Not much choice, though by then there may be a new player. Established ones are Amigos del Mar and AquaDives, with Ramon's as an occasional offering. By then Belize Dive Connection will have arrived and displaced AquaDives from the Sunbreeze dock (AquaDives are staying in the town area but going a bit further south).
All operate boats in the 45-60ft range. Amigos is probably the longest (which makes a big difference to ride) and maybe just the widest, with AquaDives not far behind on both measurements. BDC's Newton is rather shorter at (I think) 48ft, and Ramon's is a nice boat but only 40ft. All are the same style, covered in with superstructure. Occasionally the trip is made in a much smaller open boat, but I don't recommend that.
The speed of the journey won't vary much - say 2-2.5 hours each way. Unless the sea is glassy calm the ride won't be totally smooth.
If she gets seasick she'll be best staying on the lower deck and near the stern, with a good view out to the horizon. She should eat very little before or on the trip, and avoid alcohol for at least 24 hours beforehand. Something like suckable candy can help calm an iffy stomach, and it'd be a good idea to have a sweetish soft drink like Gatorade available.