I've raced in both and prefer fresh water as salt water takes a bit more washing off but it does not effect the racing, certainly in a saltwater lake.
With IC, the main problem is the cooling system. The salt, if left will (within a matter of days) start to corrode aluminium and discolour brass.
We used to carry a gallon of fresh water, then after the race, squirt at least a pint through the engine and another pint through the exhaust cooling system.
The remainder was sloshed liberally all over the boat, paying special attention to the transom area. I then used to squirt a goodly amount of WD40 over all metal and through the engine head (to stop the head bolts seizing.
This stopped most of the damage, but I still had to remove the head when I got home where I would invariably find a lovely white gel like substance forming in the water jacket (I think this is why "Coke" is used by some, as a neutralising agent)!
As long as you are prepared for it, there's no difficulty running in saltwater.
The only advantage (?) with salt water is that the boat is more buoyant - therefore goes faster
Cheers
Danny