Well it's true, at least in Canada that seaplane activity is becoming less with each passing year, there will always be a need for aircraft that can land and depart from bodies of water. Maybe in the future VTOL technology if it ever becomes economical enough could capture some of the remaining market.
Looked at in an historical perspective I would suggest that seaplanes are more popular now world wide than they've ever been.
Markets like the ones in the Maldives, Greek Islands, fire bombing as well as the Vancouver/Victoria model come to mind.
In the the thirties flying boats dominated international air travel but they were few in number and could only be afforded by governments and the super wealthy. In the same period in Canada, seaplanes did the bulk of exploration and carrage of goods and services to the remote corners.
Those aircraft were owned by airlines, few if any were in the hands of private individuals. This certainly isn't true today. If anything, seaplanes are numerically more popular and probably log more hours now world wide than at any time in their history.
Compared with land based aircraft the seaplane market hasn't grown by the same leaps and bounds but it has grown, just at a much slower rate.
When you compare the two types, floats and boats (amphibs) Floats will dominate the market share for abvious reasons. Flying boats will always have a much smaller share because they are less versatile in most respects, but if you need the ability to handle rough water and open seas than the boat is the only way to go.
As a side note... The most versatile aircraft made to date would have to be the PBY. It's duties in WWII included, dive bombing, torpedo bombing, anti submarine duties, air sea rescue, long range aerial recon, scouting, hospital and troop deployment. No other aircraft could match this. This is my opinion only and I'd love to argue my point.
Thanks to Glenn Curtis for inventing the stepped hull

It makes my job so much easier.
Cheers DR