One of Sega's Dreamcast titles happens to be called "Seaman".
It's a voice-activated game in which one communicates with a
kind-of man-fish creature strangely reminiscent of Fulchester
Rovers' star keeper Billy the Fish (viz). See eg here.

So, my question is: are Sega going to be paying for the back of big
Dave's shirt as well as the front, or is this just a bonus for them?

If you can think of other games Sega could bring out with the
same names as Arsenal players, exploiting similar "coincidences" in
amusing ways, please feel free to keep them to yourself ;-)