Hi,
Your choices for aircraft are:
A) Microlights B) Kit/plans built light aircraft (ie Light Aircraft Association - LAA - types) or C) Factory-built light aircraft ie Cessnas, Pipers Katanas, Robins etc.
Your licences are:
a) Microlight (NPPL M) b) Light Aircraft (NPPL SSEA) - from B and C above, until April 2015, and after April 2017 only from B c) Light aircraft (LAPL) - covers B & C, but only in Europe and d) EASA SEP - covers A,B &C
As has been mentioned:
if you don't have a Microlight licence you may need differences training to fly a microlight.
if you want to fly a four-seater, you need a licence other than an NPPL (M).
while there is a four-seat Jabiru from B above, all the other four-seaters are in category C. So, in general, to fly a four-seater after April 2017 you will need a LAPL or EASA SEP.
if you want to hire an aircraft, it must be factory-built, so some in A (some microlights are factory-built) and those in C. So to hire a four-seater you are looking a licence for C, which will be, after April 2017, LAPL and EASA SEP
Now we get on to floats!
There are no airworthy microlights approved for floats at the moment. Of some old types that flew, MWs and Eagles, I think there are none left.
There was a light aircraft Kitfox on floats, so a B aircraft. Based on that, it might be possible to get a microlight Kitfox approved, but welcome to the world of aviation paperwork!
There is a Cub on floats (don't know if it an LAA-type or a factory-built C of A type) and, of course, G-DRAM - a Cessna.
We have a chap here trying to get a seaplane rating on a NPPL (M). He has passed the written seamanship exam, but is no in a Catch 22 with the CAA: there are no microlight seaplanes and no microlight seaplane instructors. So he cannot get a rating!!!
My advice?
If you really want to fly a three-axis aircraft on floats:
Learn to fly a C42 Ikarus microlight. then spend £1500 at Bournemouth to add a SSEA rating to your NPPL and also get a LAPL.
Then do a floatplane rating on your NPPL (SSEA). By then, there may be a LAPL (floats).
If not, your LAPL is valid for life anyway and you keep flying on the NPPL.
In the meantime, to fly a float equipped aircraft you can buy a Kitfox, equip it with floats and use your NPPL (SSEA) to fly it. Since it is a LAA aircraft, your NPPL will keep being valid after April 2017.
Should you win the lottery and be able to buy a Cessna on floats, then you can use the LAPL - hopefully!
With the NPPL, you have a simple medical. With a LAPL a slightly more complex process and with a EASA SEP you have a Class Two aviation medical.
Finally, out-of-the-box? Get a US licence and fly a N-reg floatplane?