Before I'd consider any particular airfoil, what's your Reynolds Number
range?
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"Towed by a boat" and "..stall speed of 25mph" pretty well defines
the lower end of the envelop.

Since the thing is a water craft, apparently meant for recreational
purposes we can assume two-place, side-by-side for the load and a
stepped hull, probably with a T-tail. As with most water craft it
probably needs to be designed to allow it to be taken to where the
water is. The need for portability tends to suggest a wide-chord, low-
aspect wing, probably strut-braced to improve it's portability and
ease of set-up, which points us toward one of the low-speed airfoils
developed at Gottingen University during the 1920's... or an RAF-
series airfoil from the same era... or something from the fat end of
the 23000 series...23021 or whatever... but that would probably want a
full-span, all-flying Junkers 'aileron' for control, as mentioned by
Oliver. Brian's mention of biplane wings is a hands-down winner for
strength but becomes a mare's nest when it comes to portability and
ease of erection.

As for getting off of the water and into the air I think the real
problem is getting free of the water without the characteristic 'pop-
up.' In any case, the fact you want it to take-off and land on
water,, plus the fact it is a flying boat rather than an airplane on
floats, means it's going to have to be hell for stout if you want it
to last more than one season.

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As always, this Group can over-engineer the thing for you :-) Or you
could just jump right in, whip something out and play with it until it
breaks.

-R.S.Hoover