You've got a 60D very nice camera I had one when I was starting out. Saffer is right the 75-300 is a pretty ordinary lens, but the EF 70-300 is a lot better and very good bang for the buck goes for around $500 new. But if you want more reach its very hard to get better value for money other than the Canon 100-400 L, but if budget is an issue then the Sigma 150-500 is ok.
You can get a housing for the 60D but a decent one will cost more than the camera!
But if you want really up close and personal shots getting in the water with a housing is the only way. Panasonic, Sony and Olympus are the 3 main brands of compact cameras that are either waterproof or can fit in a OEM housing, be prepared to spend a few bickies and also to have to swim and got in close to the subject.
If you buy Sigma 150-500, a 1.4 and 2.0 extender will not autofocus on that lens. The Canon 100-400L will autofocus with a 1.4 extender but only the centre point, and will not autofocus with a canon 2.0 extender at all.
The shot below was taken with a 70D which is just a newer version of a camera that similar to yours, but to be fair I was using a Canon L prime 500mm F4 and Extender 1.4 giving an effective range of 1120mm on the 1.6 crop factor 70D (same as 60D) body. I was approx 700-800M away on the walkway at Cronulla and this guy was bodyboarding at Shark Island. Specs for the shot are F5.6 Iso 640 and shutter 1/3200th and the best advice I was given was to invest in good quality L series lenses, and even to avoid the Canon EFS lenses as they only fir the crop bodies (70D and under).
Also this is a new Nikon thats fully waterproof -
www.cnet.com/