Yep, those mechanisms and several more, including:
1. RFI from your onboard equipment/instruments getting into the receiver either by radiation or conduction thru existing wiring
, including power cables
2. Radiation from the intended RF ground or counterpoise
(that radiates, too), getting into other instruments aboard;
3. Poor or degraded connections in the antenna/ground system;
Point is, as you know, you need to separate the RF ground -- whatever kind -- from all other grounds on the boat and, insofar as possible, from the proximity to other wiring
on the boat.
Physical connection to the seawater (thru-hull or grounding plate or whatever) isn't a bad idea -- and in some cases it may help bleed off static electricity and reduce noise
-- but it isn't strictly necessary in most cases.
Nor is grounding of the transceiver itself (apart from the 12VDC power lead to the batteries). That often creates ground loops.
Of course all the books
and instruction manuals
will usually tell you to ground everything to everything, and just use lots of ferrites or RF chokes.