OUr "go to" operator is Bird's Underwater, but we have used River Ventures when Bird's is full (Bird's referred us to them). Their guides/captains are excellent and the shop folks are really nice as well. HOWEVER, there are some things you should be aware of:
GENERAL THINGS:
It has been pretty warm up there, manatees will probably still be around but not as thick as earlier in the year.
Get the early (6:00 am) trip. This is BY FAR the optimum trip for interactions as you are the first divers of the day and the manatees are not worn out with divers or have not moved out into the Gulf to feed. Later trips in the day might be better conditions for photography, but there will definitely be fewer manatees. Even more, if the weather has been warm, the early trip is far and away the best.
SPECIFIC THINGS FOR RIVER VENTURES
1. I shoot photos with a high end rig (OM-D). We showed up with our own gear which was appropriate for what we wanted to do (7 mil suits, boat coats, hoods, etc for maximum warmth for a long time in the water) and we were plainly experienced divers. BUT they sought to enforce a "no fins" rule for all people on the boat (ie lowest common denominator theory). I explained that it was impossible to maneuver in an efficient way with the camera without fins so as not to flail my arms but they seemed to take a hard line. Before we got to the site, I talked the captain about our experience and persuaded him to let us use the fins while he observed to see that we were not idiots and he agreed. After 10 seconds he let us go on our way, fins intact. So, if want to use fins, plan to discuss this with them IN ADVANCE and maybe take a log book to show your experience.
2. Bird's (and most ops) run a 6:00 am and an 10:00 or 11:00 am trip. This means that you usually leave by 6:30 and are at the first site by sunrise (no one in the water before sunrise is one of the rules). This gives the early trip a couple of hours in the water and the chance to try a couple of sites (hence the boat coats for when we are moving between sites) if for some reason the first site does not pan out. River Ventures, however, runs a 6:00. 9:00 am and 11:00 trip. This means that, by the time you are at the site, you have just over an hour in the water. This is MUCH less than we have had with Bird's, as the pressure to return early to the dock for the next trip causes the captain to keep everyone close together and chase you down for the return trip. If you want a lot of opportunity for interaction, or time for video and photos, this is not a good thing. I assume they have adjusted their times for daylight savings since sunrise is now an hour later, but the pressure of that mid-morning trip will still keep your in-water time low.
Our trip with them was nice and the manatee interactions were great. The captain prevented any misbehavior from the snorkelers and let us do our own thing to the extent the time allowed. Part of the problem may have been that it was President's Day weekend and they were expecting an inexperienced mob and had little inclination to make exceptions to the no-fins policy. Also, all of their trips were booked solid. Maybe if they do not fill the 9:00 trips, the early ones can stay out longer. But, check with them about this before you go.
I am not "not recommending" them, but you might want to check with them about these things before you go. If they still do the three trips, you might want to check around with other ops if you want more in-water time.