take off on land and land in the water (or vice versa)
Nope, sorry. Can't do that.
Mine used a 61 two stroke and I have flown several others with 61s. Your 65 will be more than adequate. Adding a larger engine will add more weight and require adding more nose weight also. I can not emphasize enough that you do NOT want to add any weight to the Seawind that you don't absolutely have to.
Jim