You can download a .pdf copy of the ARRL Antenna Book on the web - it is a great resource and highly recommended. Hams definitely know a lot more about antenna design than boaters. My basic understanding (I may be wrong) is as follows:
1. On a boat
you want to use a vertical antenna with seawater as ground. Seawater is a perfect ground, so you definitely want to leverage this free resource.
2. The antenna radiates perpendicular to its axis but the angle of radiation is very broad, say 160 degrees. So, on a boat most of the radiation goes towards the horizon (which is what you want, for DX at least) and the part that goes towards the water
gets reflected as well, which is good.
3. All the other antennas, dipoles, directional Yagi's etc. need a lot more space and also they need good separation from the ground (sea), i.e. 30-40 feet. You can imagine this becomes impractical on a boat. Also, it does not get you anything useful. A lot more people will be using verticals on land if only they could have good ground. We have a perfect ground at sea, so we should use it by means of a vertical antenna.
4. The rig definitely complicates things but it is too complex to worry about. Just try to get as much separation as possible. The most important thing though is to have the antenna tuner as close to the feed point of the backstay as possible. Two feet is ideal. Most of the energy radiates in the beginning of the wire, so you do not want to waste this.
5. In the end, the antenna/ground may not be as important as we think. Clearly, the better the antenna and the more solid the ground, the more efficient the radiation pattern will be. In the old days of voice communication you needed a good signal to noise
ratio to be heard. These days people use mostly digital modes (PSK31, JS8, FT8). These modes have 12-15 dB advantage over voice. Even CW is a lot more efficient than voice. Once you get away from the RF noise
of the marina, have relatively OK ground, a tuned antenna and well calibrated soundcard or Pactor modem
, you will be in much better shape than most hams on land.