We had a single 250HP OX66 on our 270 Islander when we purchased it new. Did a lot of prop work before we repowered to the twins.
First off, I think you should be running a larger diameter prop.
The OX66 max RPM is 5500, so you are currently UNDER-propped, as shown by your 5700rpm WOT.
I would recommend you first try a Yamaha SWS 15.25" x 17". This is what we had originally on our OX66, and it should drop your WOT RPM down to about 5500.
You might also consider a PowerTech OFS4 prop, in 17" pitch. This will also drop your RPM as needed, but will give you a LOT more bite, with greatly improved hole shot and the ability to hold plane at a lower RPM. This is the prop that we had on the OX66 until we repowered. (And we have PowerTech OFS3 props on the F150s now).
You should be able to find a good prop shop in Morehead City. Let them know that you are seriously looking for a better prop and ask if you can try a few. If not, then maybe you can work with my prop guy at Bay Propeller in Chesapeake, VA.
Brian