As Steelspike states, you really need to do a lightly loaded test because the real determination of the right prop for your setup is your RPM at WOT.
THAT HAVING BEEN SAID...I had a similar setup to yours (15' tri-hull with a 1976 70 HP Johnson, usually ran with 4 of us - two adults and two smaller kids), and I had a 17" Pitch Stainless Steel OMC 3-blade prop, and a 19" pitch aluminum OMC prop. Both spun up into the recommended range (about 5600 RPM), and was enough to get tubers and/or kneeboards in, but didn't have enough holeshot to get skiers up reliably. Knowing what I know today, I would've tried a 15" 4-blade prop if I really wanted to do watersports. A decent "starting point" if you do need a prop is a 19" three-blade aluminum. I found the Solas Amita props to be very good all-around (I had one on a previous motor I had on the 15' tri-hull and am currently running a 4-blade Solas Amita aluminum prop on my Mercruiser/Alpha 1). Price is between $100-$125 here on iBoats.
Rgds