Here are some choices for marine antennas:
Click twice on each pic for full resolution. Start with the multi-color table on the upper left corner.
FYI, the best antenna you can put aboard for DX on 20m is a single-band VERTICAL dipole. Fed at the center with RG8X coax, you just hoist one end and tie the lower end off near the deck.
TRUE STORY: in the Virgin Islands a few years back I had my FT817 on my chest while relaxing in the cockpit at anchor. It was attached to a 20m vertical dipole on the bow. I made 2 DX contacts with 2.5watts (internal batteries) on SSB: a station in Armenia (gave me a 59 report) and a ZL in Aukland NZ (gave me a 57 report).
For QRP, especially, the antenna is EVERYTHING.
Yes, you can rig all manner of whips and such, but with only 5 watts you won't be very happy.
It makes no sense to put a vertical atop the mast. For HF verticals -- as contrasted with VHF verticals -- you want them mounted low down, close to deck in order to get the best low-angle radiation.
A radial for 20 meters is only 16' long. Use any kind of insulated wire.
With the dipole, you don't need radials or any ground at all.