You play as an old fart* who, upon seeing that his dolphin friend has been kidnapped by an oddly huge and extremely well-equipped group of... dolphin-nappers, concludes that the reasonable response is
There's really not a huge amount to say about the gameplay, which is what I've seen described as a rail shooter, i.e. you can't actually move and the only way you can avoid getting hit is by shooting things before they reach you. You start with a boat with a single measly gun and can upgrade it and obtain new boats; eventually you end up with the boat pictured above, which can heal itself and has a plethora of autofiring weapons (even the basic boat's gun can be upgraded to autofire). In fact, once you've upgraded the boat enough, if you just point the gun in the right direction and then go to another tab/window for a while, the strongest boat can go about two-thirds of the way through the game without further guidance. (It is admittedly not a very long game.)
The main draw, really, is the ludicrousness of the premise; this implacable old man is directing a massive amount of munitions at a bunch of generic bad guys and somehow managing not to injure the dolphin that they've trussed to the back of their bizarre battleship.
So, well, really just something you can spend an afternoon on; I don't know that it would take a good player even that long.
*Actual quote from the game's description: "As a kid, your best friend was a dolphin, some bad guys kidnapped it and now, while you are older, you are going to fight them." Now, while you are older. That's just kind of a funny way of putting it.
**Dolphins have a reputation, of course, of being nearly intelligent as human beings and also of being pure, wonderful angels of the sea. In truth, dolphins aren't nearly as smart as they have a reputation for being (they're probably not any smarter than dogs, it's just that they can talk to each other-and tool use has been observed in very simple animals such as wasps, so communication most likely doesn't necessarily require a lot of smarts either-in fact, bees have a pretty sophisticated language too, so...) and are horrible, horrible creatures (known to kill babies to make females come into heat, known to rape, and known to assault or attempt to assault humans as well as basically everything else-they're probably as or more likely to attack you as any random shark-and are known to kill just for fun rather than for any particular good reason***).
...It's possible I'm using this post as an excuse to rant about dolphins.
***One could almost argue that this is their most human trait, though seemingly wanton slaughter is more common in nature than popular culture would have you believe.