I would beg to differ that the frequency of TV and Marine VHF are "similar" This is true only if you mean that 80 meters is similar to 30 meters. Perhaps if they where harmonically related (80-40-20 or 30-60-90) but your tv reception from a marine VHF antenna is going to be pretty weak at best. Add to that that digital transmissions are spread spectrum (VERY wide bandwidth sharing the regular TV channel spectrum) and you will be well served to have a dedicated TV antenna
OR
convince your first mate that sailing is fun and tv is boring........ oops can't get there from here (YET) ;-)