cobra triangle is probably the only game that involves driving around in a boat that is actually fun.
And then there are the upstream levels. Obviously you'll be driving the boat upstream while avoiding various debris and swirving through moving whirlpools. These levels are a bit easier than the waterfall levels in my opinion, however, you have to watch out for the whirlpools which can take you down in one hit.
There are also bosses in this game which include different sea monsters such as the serpent as pictured on the box art, a giant crab, a squid, and the final boss which is a shark. I don't think I've ever actually made it to the final boss without the help of a game genie. These boss battles are pretty well done and each boss is a memorable one and has interesting music. The crab's is my favorite. As it starts to dance the music picks up. hehe.
And I can't forget the infamous "save the people" levels. you start out with 8 strange looking people treading water in a square in the center of the stage. Your job is to protect them from the enemy boats and flying saucers (yes, there are flying saucers...) that try to take the people away, most likely to be made slaves, or to be tortured since i don't think they have legs. The first save the people level is pretty simple to beat and saving them all isn't too difficult a task, but the second save the people level can be pretty challenging if you haven't had much practice. The flying saucers carry missiles that can render you immobile for a short period of time, which is all they need to carry off one of your persons.
So, overall I'd say Cobra Triangle is a pretty solid game considering its age. Hopefully it'll be on the virtual console one day so more people can experience this great game.