I have seen several pictures of Chinooks landing on lakes (but have nowhere to host them so can't post them), including one of a RIB coming into the back, looks quite impressive taking off with all the water gushing out.
I believe all the pictures I have seen have been on lakes, not the sea, ie fresh, not salt, water. I seem to remember that salt water can be very damaging to aircraft, but fresh water isn't so bad, am I wrong? (it has been known to happen)