I am a Ham hence I don't use the marine SSB
Look at this for main marine SSB
frequencies more specifically the Marine distress/DSC and the ship-to-ship frequencies.
The distance you want to cover (and type of journey you are undertaking) and the propagation will determine the frequencies you want to use.
Ask around on this forum; people will be able to indicate which ones to use and also the marine SSB nets (like 12.359).
Frequencies around 12 (marine SSB) -14 (HAM) Mhz are very good all-round long distance frequencies (thousands of miles) (good atmospheric propagation conditions most of the day)
For me HAM 20m band (14 Mhz) and 17m (18 Mhz) are the best long distance (DX) frequencies on board. HAM 15m (21 Mhz) and 10m (28.5 Mhz) really depending on atmospheric propagation. My wire antenna
system is 9.2m total length and this ensures good long distance use until 15 m HAM band due to the radiation properties and angle (linked to maximum antenna
If you want to use mainly frequencies below 10 Mhz a longer backstay/wire length 40-45 ft will favour this (better efficiency and radiation patterns); if mainly above 10 Mhz till 25 Mhz I would suggest something around 30 ft.
The longer antenna (40-45 ft) will still work on higher frequencies and be tuneable off course (if one stays away from 1/2 wave multiples) but hte radiation pattern becomes less optimal for long distance communication...but this is not saying "impossible" for long range DX.
THe shorter antenna will become increasingly in-efficient into the 4 and 2 Mhz frequencies.