Gargoyle Games‘ isometric adventure Hydrofool (1987) is different because the whole game is set underwater.
Rather than walking and jumping, the main character – a robot called Sweevo – must swim around and explore the environment (a giant aquarium called The Deathbowl), looking for plugs, which he must then pull to drain the water away. The plugs, however, must be pulled in the right order. There are also other creatures swimming around that are a danger to Sweevo, which you must avoid.
And – if that wasn’t difficult enough – Sweevo begins to slowly rust as soon as the game starts, so you’re up against a time limit as well.
Hydrofool is still an enjoyable ZX Spectrum game to play now. It is also the sequel to Sweevo’s World. So is something of a curve ball.
More: Hydrofool on Wikipedia