Hydrofool was developed by Gargoyle Games and published by Faster Than Light in 1987. It’s the sequel to Sweevo’s World which was released in 1986.
It’s an isometric adventure/puzzle game set underwater, in a gigantic aquarium known as “The Deathbowl“, which has become so heavily polluted that the only remedy is to completely drain it by pulling out each of its four plugs. You play as Sweevo who can swim around, pick things up, ride upwards on bubbles, and downwards through whirlpools. The idea is to find four items to release each plug, although you can only carry one item at a time. You must make your way around a large number of interconnected screens, avoiding hostiles, and finding those required items.
A time limit is imposed due to Sweevo rusting in the water, and this is indicated by an ever-decreasing progress bar. Contact with hostiles or some seafloor structures will increase the rusting damage, but you can find and pick up oil cans to remove some of the rust.
Just like its predecessor, Sweevo’s World, in Hydrofool you can choose where to start from the title screen (by pressing the A to D keys).
The 128K version of the game features some jolly music by Rob Hubbard (although this only plays on enhanced Spectrums). The 48K version has a title screen tune, but in-game there are just spot sound effects.
Hydrofool is a neat little game and at least tried to do something different with the isometric format at the time, and it succeeds to a certain extent.
More: Hydrofool on Wikipedia