As someone who picked this up before playing the game I would say it's worthwhile IF you're planning to 100% complete the game. But that said looking at the price it can appear a bit steep, and really is looked at as more of a way to give the developers a bit extra.

The extra in game content is in a locked shed (this is the key for the lock). It gives a charm that counts as a 1 slot fishing rod, and a 1 slot engine. These two items if collected at the beginning of the game will speed up your playthrough by a significant amount, in both short term and long term.

In the short term you have a 1 slot engine that can be installed alongside your default engine, thus allowing you to travel a decent bit faster from the get go. As such it takes less time to get between fishing spots and docks. Meaning you can effectively have a larger range that you can fish in and make it back at the beginning. Eventually the engine will be outclassed by the top end engines that you can research in the game. So eventually you'll be able to sell this item or keep it in storage with other similar looking items.

In the long term you have the 1 slot fishing charm that can be installed alongside your fishing equipment. From what I've read this give a +5% chance of getting the special "aberrations" of fishes. Although it doesn't seem like much the default rate is 1-3% so boosting it to a 6-8% is significant. That said there are times where you'll see spots that offer a 100% aberration. However those only have a given chance of spawning on the fish spot you want. The majority of aberrations you'll catch will come from one of these 100% spots, however not all. Furthermore 1 fish in particular can only be caught in an Oceanic Trawl net (no fishing spot) and it has an aberration, so at the very least this will likely decrease the time you'd have to spend trawling the open ocean.

However that said it's not all positives with this DLC.

Firstly this DLC is of a diminishing return. What I mean by that is the longer you play without it, the less valuable it would be when you get it. As in, if you've already made substantial progress on upgrading your engines depending on how far along you are you'll either get very little use from the engine it gives you or even none if you've already done all engine research. On top of that if you've already been fishing and captured a good chunk of the aberrations, then the charm that helps find aberrations will get less use, purely because you have less spots left to visit. Also a spot where you'd still need an aberration could have given it to you earlier as you wouldn't need to be as lucky to get the aberration.

Secondly if you judge games on how much money you spent on it vs how much time you spent in game then this DLC is definitely not for you. As by that logic this increases the cost and decrease time played which is normally the reverse of what people judging games by this metric want. Not everyone will use this metric to judge games, however if you do judge by that metric you should skip this DLC and have a longer start of game and a longer grind for aberrations.

Overall I would definitely recommend this if you haven't played the game much but would want to.
It would be pretty hit or miss if you're halfway through the game, depends on how you feel.
And if you're at the end of the game you probably won't get use out of this and should avoid it unless you just want it for collection.

As someone who picked this up before playing the game I would say it's worthwhile IF you're planning to 100% complete the game. But that said looking at the price it can appear a bit steep, and really is looked at as more of a way to give the developers a bit extra. The extra in game content is in a locked shed (this is the key for the lock). It gives a charm that counts as a 1 slot fishing rod, and a 1 slot engine. These two items if collected at the beginning of the game will speed up your playthrough by a significant amount, in both short term and long term. In the short term you have a 1 slot engine that can be installed alongside your default engine, thus allowing you to travel a decent bit faster from the get go. As such it takes less time to get between fishing spots and docks. Meaning you can effectively have a larger range that you can fish in and make it back at the beginning. Eventually the engine will be outclassed by the top end engines that you can research in the game. So eventually you'll be able to sell this item or keep it in storage with other similar looking items. In the long term you have the 1 slot fishing charm that can be installed alongside your fishing equipment. From what I've read this give a +5% chance of getting the special "aberrations" of fishes. Although it doesn't seem like much the default rate is 1-3% so boosting it to a 6-8% is significant. That said there are times where you'll see spots that offer a 100% aberration. However those only have a given chance of spawning on the fish spot you want. The majority of aberrations you'll catch will come from one of these 100% spots, however not all. Furthermore 1 fish in particular can only be caught in an Oceanic Trawl net (no fishing spot) and it has an aberration, so at the very least this will likely decrease the time you'd have to spend trawling the open ocean. However that said it's not all positives with this DLC. Firstly this DLC is of a diminishing return. What I mean by that is the longer you play without it, the less valuable it would be when you get it. As in, if you've already made substantial progress on upgrading your engines depending on how far along you are you'll either get very little use from the engine it gives you or even none if you've already done all engine research. On top of that if you've already been fishing and captured a good chunk of the aberrations, then the charm that helps find aberrations will get less use, purely because you have less spots left to visit. Also a spot where you'd still need an aberration could have given it to you earlier as you wouldn't need to be as lucky to get the aberration. Secondly if you judge games on how much money you spent on it vs how much time you spent in game then this DLC is definitely not for you. As by that logic this increases the cost and decrease time played which is normally the reverse of what people judging games by this metric want. Not everyone will use this metric to judge games, however if you do judge by that metric you should skip this DLC and have a longer start of game and a longer grind for aberrations. Overall I would definitely recommend this if you haven't played the game much but would want to. It would be pretty hit or miss if you're halfway through the game, depends on how you feel. And if you're at the end of the game you probably won't get use out of this and should avoid it unless you just want it for collection.