u/lycanreborn123 was a bad answer, only Sea Turtles have flipper-thingys.

A decent way to guess is that turtles generally are a bit flatter, while tortoises look pretty dried out and dusty, and can have more domed shells. Tortoise, tortoise, turtle, turtle.

Different dialects of English draw different distinctions though, so it barely matters. Tortoises don't live in water though.