Went through this scenario a little while back.
The WS 650's are night and day better than the Rockford OEM. Much, much more mid bass, more output and an all around better built speaker. The RF M-series are not bad speakers and I would say that if your boat already had them in it, put some good power to it and see how you like.
But since you are at the crossroads of doing new speakers. I'd say go the Wetsounds. Yes they are more money but you will have no regrets. With the RF speakers - you may play them a couple times out and they may leave you wanting more.