The problem you quite rightly said is the salt water. It gets in everywhere. You need to have the space to get the aroeplane out of the water to regularly hose it down with clean water.
A cub is fine if it is just for you to play around with, although a bit small. A 172 tends to be underpowered, however a C180/182 or ideally a C185 is an ideal combination of size and power.
On the sea you have to be very careful of waves and worse is the swell, particularly in a a/c like a cub or even a C172. in fact I would say no to that.
If you get amphibious floats you can then land on water and drive off up your ramp to wash the aeroplane down.