You can find Polk Audio on Amazon.
The DBI 6.5 " or 5.25" series are Marine certified and sound great. If you add an amplifier, they will blow you away.
But....let's talk about how much you want to spend and how good of sound your after.
The Polks are great, but a wake tower enclosure will cost more than the Polk speaker. Appx $ 89 for 2 tower cans plus another $58 ish for a pair of decent 5.25" or 6.5" Marine rated Polk. Did I say amp.
You can get preloaded tower speakers cheaper, but you get... well. If you just want music and maybe hear it while riding, you can pay less for a Boss or other budget tower speakers.
If you want great out of the box sound and quality, go Polk with Rockville cans. Then add a small amp and you will make a great sounding budget UTV system.
Opinions will vary by ownership...
Look at Azon offerings for wake bar tower speakers.
Pay attention to higher RMS ratings that are CE certified(real) and speakers that are IP67 marine rated.
The Boss head is really low budget but it does put out about 15 ish watts per channel and enough to make 4 speakers sound ok, without an amp.. did I say amp.
I know, too much.
I also have 6.5's on motorcycles with amps you can entertain for a half mile.
Polk are good quality and affordable.
*** Rockville Wake Tower Speakers are often Polk speakers in Rockville enclosures. Might be a good and affordable option.
You could also use 5.25" speakers with that Boss or similar head unit.
Polk's component speakers are higher end, better sound..
Ok I'm done.
Search stereos on the forum and hold on for a ride