The Reef Knot, is a basic knot with the purpose of joining together two ropes of equal thickness in a flat and non-high strain situation.
The word Reef, gives away that it is an essential knot in marine and diving situations.
The clove hitch is another very easy to tie knot. The purpose of the clove hitch is to secure ropes to trees, posts and other secure things such as pipe.
The sheet bend, is probably the best knot to know when tying ropes of unequal thickness together. unlike the reef knot, the sheet bend does not slip under tension and is unlikely to jam (so it is easy to untie)
The bow line is the quintesinal diving knot which all commercial divers need to be proficient at tying. similar to a figure eight, the knot has an eye at the end of the rope. The bowline also has the benefit of being both easy to tie and untie (even after being subject to a heavy load)
the figure eight knot is an important yet very easy knoto to learn. it is the basis behind many more diving knots (e.g figure eight bend, rethreaded figure eight) and used correctly is a fantastic stopper knot in the water.
(the link takes you to the rethreaded figure eight link)