First note, that despite the very suggestive title, this is not associated with the game Ship Simulator Extremes that you can also buy on steam, and is developed by a different company. This is both a good and bad thing - it's good because there are many, many fewer graphical bugs and general slowness than you get from the classic ship simulator, but it has problems that I'll get into later.
My biggest concern with the game is the ship physics. They just don't feel right - a long endurance rescue vessel seems more like a small boat, and the boat feels terrible at low speeds, because of how it moves in the wrong direction when trying to steer. This might be realistic, but it sure doesn't seem like it.
Secondly, there's a distinct lack of content. There are only a few other ship classes, and cargo ships are represented by fishing boats etc. It gets annoying, but not too badly. Given the nature of the ships that they've modelled, a true open ocean scenario would be wonderful.
Third, the towing and cable attachment walkaround system is way, way too slow, even in calm seas.
I actually rather like this game. Poor ship physics aside, the campaign is at least okay, and there is a fair amount of diversity. However, it isn't quite worth 25$ - if it was 15$, I'd say you should buy it, but not at the current pricing. I hope that the developers continue to expand the content that's avaliable, though I'm not too hopeful.
Edit (too long for a comment):
I think that the easiest one of my complaints to fix is the walk-around one, I suspect that there's just a constant somewhere that can be changed.
The motor launch dynamics also need to be improved, as the sliding action when attempting to turn at low speeds makes it difficult to perform rescues effectively. However, this might be a realisim thing, as I have very little real experience with these boats.
The thing that I'd like the most to be added, content-wise, is an open-ocean component, far distant from the shore. The Hermann Marwede in particular was designed for this, and it doesn't show up a lot in the game. This would also go well with the addition of the emergency towing vessel Mellum, also based on Heligoland.
Only one real new ship class is needed for freighters, really, and I think that something like a 5,000TEU containerized vessel would be enough. I'm not sure how hard it would be to add, though.
With respect to the missions, something that's been bothering me about the ones that exist already is that they're so close to shore and in good enough weather that the majority would seem to be done by helicopter. It would be nice to make you cooperate with a helicopter, possibly even conducting air operations off of the Marwede.
Finally, a mod kit would be very nice. VSTEP doesn't have one, for reasons I can't determine, but I know that I would be interested at least in developing new content for the game.
I think that implementing any two of my suggestions would make me, at least, reccomend this game. I'm excited to see where the game goes in the future.
The ship physics are really the worst thing, though. The ships's weight more or less does not exist (the RHIB pushes a 5000 ton freighter around), and the boyancy model is nonexistent. Floating appears to happen based on a uniform height above the water. Furthermore, the drag model doesn't work: when you have a ship under tow, the top speed does not decrease whatsoever.