Thank you for the info Fenlander!
I may be wrong but according to my research the 420 is also wider so I would assume the seat is wider as well. Here's a comparison (the table was google translated from Greek that's why the labels are a little weird in some of the columns)
I considered a second seat but the dimensions of the gear we carry would make it impossible to fit it below the two seats, some stuff (eg the spearguns) are halfway in the boat/halfway resting on the tubes, and if I install a seat there then the guns would pretty much be in my lap at that point which would make for a very uncomfortable ride
Secure storage sounds good, my concern though (on top of the reduced amount of space) is things like diving fins/weight belts/whatnot all sliding down to the middle narrow part because of the v-sides of the bottom, resulting in even less useable space in the end. Which would become even more of a problem if I sit on a bench in the back because all these would end up right below or on right top of my feet. And I can't really see a good way to keep them in place on the sides of the floor..