This experience was incredible and I highly recommend it! I went on the 6:45am VIP small group tour, there were two other people in my group so we had plenty of room. When I called a few weeks prior and spoke with the team who works there, they were very helpful and gave advice on the best days and circumstances (weather, time of year) to come do the tour. They are really invested in giving you the best experience possible.
You don't have to bring anything but yourself, a swimsuit, and a towel. Also bring a change of clothes because you'll be freezing after! I really appreciated the heated boat because my body gets cold really easily and my lips were purple at the end! But it's absolutely worth it to go after a few cold nights, you'll see more manatees that way. The manatees are worth the cold! They provide you with wet suits, snorkel, and a mask.
Our group saw eight manatees and spent most of the time floating/swimming with a mom and calf. Our tour guide and captain were great and gave us a special experience, we were all new to snorkeling so I was very glad for their help! One thing I really loved about River Ventures was their photo/video packages. During the tour they take photos and videos of the experience and you, so you only have to focus on enjoying the time you have. If you don't have an underwater camera, or even if you do, I recommend spending the money so all you have to do is enjoy the experience.
I will definitely be back in the future! Thanks River Ventures.