That's the OP's problem not mine, the issue is that the ones who have posted gives a you know what about wot rpm which isn't known so far and a prop maximization will better what the OP's wants to achieve. Better doesn't mean Top Performance.
Happy Boating
The way I see it is that your lack of experience is hanging out.
The rest of us know that what the OP's asking for (5mph) is beyond that motors ability. That's what we are talking about. RPM and prop information simply not required to know that.
That makes prop and rpm data irrelevant. It IMPOSSIBLE to achieve what the OP want's to do as no matter what rpm he's turning, or what prop he has on it, that level of performance is out of reach - with a 60hp pontoon boat.
Speeds in the low 20's while pulling a tuber with 4 people aboard? Not going to happen. It's that simple.
Telling the OP he can put some type of prop on it that will allow him to do what he's suggesting is questionable advice. That's just as simple.....
Stay safe...