I have a Lazer II 21p ss prop on a Mercury Opti 115. I bought a Merc 125 Opti that was on a Blue Wave 20' center console. It has a really low pitch. The seller thinks its a 13p? I am just looking for a better hole shot and don't really want/need a lot of top speed. What would be optimal setup? I have a 2006 Nitro 482, it's 18'9".

Good questions, not enough details.
Your opti is a hard motor to prop, it's power curve is narrow and it's pretty gutless on the low end, sorry to say.

AND...it's got a hard rev limiter at 5,700 rpms, you don't want to hear that thing screaming at you all the time, so the details you'll need to share: Whats your current rpms with the lazer at WFO? THAT number will say what you really need to be running. It won't be that 13, not gonna work at all.

I would try that 21p Laser II on the 125 before I would do any thing else, use that as a starting point to get WOT rpm and holeshot info and dial in from there....that prop might be able to be tuned to work.

If the prop is not marked, take it to a shop and let them tell you what it is. If you don't know what you got, you don't kow what you need and you can guess till the cows come home, throw props at it trying to get it right, and end result is that someone will have made a lot of money selling you props and you will still be trying to get your boat to run at it's optimum.

Thanks guys!

Thanks,
Won't have the 125 on until Monday. I'll get that info after i drive it. Probably have the 21p put on when we do the motor swap...

Did you do a compression test on that 125? 13 pitch sounds really low almost like it might be covering up a weak cylinder.

Not really. A 13 heavy cup flats prop would be about right for that blue wave.