The historical Romans had naval battles in arenas like the Coliseum --
https://en.wikipedia.org/wiki/Naumachia#Early_naumachiae
That part didn't bother me. That part looked cool. Some of the goofy CGI bothered me, though. Having watched the original again, it
greatly benefits from its commitment to practical effects.
I felt worse after the second trailer compared to the first trailer.
1.) The only returning character I've seen so far was Connie Nielsen. I think Derek Jacobi (the Senator in the original one) might be making a return, but he's 85 so probably a cameo at most.
2.) You hit on what has me concerned all the sudden. It is a new cast and mostly new characters, sure, but I fear it is going to be a complete rehash of the original in terms of the plot.
"Same plot but bigger and badder effects!" can work (see
Mad Max: Fury Road compared to
The Road Warrior, though that's a shallow comparison) but usually produces at best a "meh" sequel.