nice to see another new cart owner here, and yes, outdoor stuff, boats, motorcycle, carts an all prolly better off with a marine style built in if you go that way, not just for water, but dirt, moisture, sun and mother nature in general...
here's what I'm looking at for audio on my cart...
https://jawbone.com/speakers
https://www.ihomeaudio.com/iBT4GC/#support