There has been a thread on this in the past, but the main differences I've noticed between an auto to the ground on skids vs. to the water on fixed floats (in Bell 205 and 206) are:
1. Big pitch up on entry, presumably because of extra surface area up front, which needs a good amount of forward cyclic to counter;
2. Yaw instability in auto, just like the waffly feel in forward flight;
3. Flare height and level normal, but may be hard to judge especially on a glassy day;
4. Actual water entry is quite relaxed and 'spongy' - just make sure it's straight, and a level to slightly nose up attitude, and expect a bit of a bow wave with any forward speed - can wash into the cabin if the doors are open, quite sporty.
5. Hover / taxi autos are very forgiving collective-wise, but don't be moving sideways when you touch down.
I haven't autoed into much in the way of swell or waves, but have been told to land into wind on a small wave day and along the swell if the waves are significant - seems to make sense.