Yes, grounding (counterpoise actually) is a difficult issue for marine SSB. Modern ATU's - Automatic Antenna "Tuners" can match the proverbial bedspring. A big advantage of maritime hf is that the salt water surrounding the boat makes a good counterpoise and the noise level is only what what you create on board.

This problem is most difficult on power boats and multi-hulls. On ballasted sailboats in you can tie your ATU to the keel you should have a good counterpoise even with internal ballast. Sintered bronze shoes are very small in surface area and are not as good as they claim. They were originally made as grounds for Loran receivers. Another snake oil product is the bundle of wires concept. I forget the name but it is a cable of different length wires that you lay in the bilge. This has been analyzed to death on the SSCA discussion board.

Another issue is getting an antenna long enough to use the lower frequency bands. Most backstay antennas won't work well on 4 Mhz bands. 2 Mhz marine SSB frequencies are almost impossible.