I just watched a video of seaman and was horrified!!! haha

You already own the game I'd have recommended most.

Other than SoulCal, though, I recommend:

-Shenmue/2
-Sonic Adventure/2
-Jet Grind Radio
-Phantasy Star Online
-Mortal Kombat Gold
-Power Stone/2
-Guilty Gear X

That's about it.

powerstone 1 and 2, shenmue 1 and 2, skies of arcadia, grandia 2, samba de amigo, rayman 2 (the graphic was gigantic at that time), jet grind radio

"Seaman, Seaman, Seaman!" and the microphone for the DC is very cheap to come by.

Panzer Dragoon Saga

I guess Shenmue lol, but why buy a dreamcast in this age?

Crazy Taxi 1+2- Just classics. The ultimate hardcore casual games.
RE 2+3, Code Veronica (get X if you can)
Panzer Dragoon Saga
Rayman 2- Still amazing
Jet Grind Radio- Still looks great.
Ecco the Dolphin- Forgotten gem.
GTA2: old school GTA
Power Stone 2
Ikaruga- addicting shooter.
Headhunter- hard to find, looks great and unique gameplay.

If you can live with 1999/2000 graphics, don't forget OMIKRON the nomad soul ( Quantic Dream ), one of the best games EVER.

You should get Bleemcast! so you can actually play some good games on it.

Am I right in saying that the Dreamcast was incredibly easy to pirate?

Literally, you would just have to burn the game to the disc, and it would load?

Not that I usually condone piracy... but you won't be able to buy anything first hand, nowadays, anyway... and I'm sure second hand games are even particularly rare.