The Polk MOMO is a bit undersized as a 6.5" versus the present Polk MM series. By comparison I would call the MOMO a 6" if I was getting technical.
Most 6.5" coaxials are 6.875" grill O.D. and the Wetsounds XS-650 spec is 6.8" so just a 16th under the average.
If I recall correctly, if some of your Nautique speakers are recessed into the vinyl bolsters then most any and all 6.5" can be a very,very snug fit but will usually go without any mod to the bolster and without chaffing the vinyl on the way in.