Fun1.5 or other
My 2 cents: Go with something other than the Fun1 - unless you're only looking for an easy to roll pool boat. Suggestions would be: JK SideKick, WS Fuse 35, LL Remix 49, or a Fun1.5.
The Fun1 will be the easiest to roll in the pool, but any of the other boats will handle better on the river. The stern of the Fun1 (and Fun1.5 and all original classic Funs) is "grabby", so the closer your child is to the "optimal" weight, the less forgiving the boat will be on the water.
The Fuse35 is noticeably more difficult to roll for beginners but it's wider, more balanced, and more stable in the river. The SideKick is also a much better river runner than the Fun1/1.5.
This summer after plenty of trial and error I sold my boys' Fun1 and Fun1.5 and they now use the Fuse35 and SideKick instead. The Fun1 was great when they were little (2-5) in the pool/lake, but not as much now that my youngest boy is 6 and weights in around 65 lbs.
I had a Fun1 for years - it was mainly used in the pool and lake and was great for little kids (2-5 yrs) - but when my six year old took it down the local class II/III river this summer (sans skirt) he swam way too much.