Just thought I would chime in... New diver here, dove with both DipNDive on the West side of Grand Island and Discover Diving on the East side - think they tend to stay to these sites (unconfirmed). Now I haven't seen a ton of interesting things underwater yet as I am just getting started but I LOVED the drift dives. Both were in the summer - wearing my only wetsuit - 7MM jumpsuit. Perfectly comfortable - river temps both times were about 70-73. Max depth is in the high 20s - but you might find a few holes.
RHWESTFALL made an good comment about the West side. As a new diver the channel from the piping laid across was a really cool experience. Divemaster knew based on time of dive that it would be coming up. Signalled to us via pre agreed upon signal that the channel was approaching - we sort of crawled in and the current almost stopped. Looking up you could see seaweed and other "treasures" whizzing by. A cool feeling to say the least. They had explained the exit of the channel - the water swirls on the outgoing lip and exiting is less graceful then entering the 8foot wide channel. The current at the exit lip gives you a nice push into the ground. FULL GLOVES and skin coverage are a must. I had a fingerless glove which I find useful for dexterity but now learned its quite useless on this dive. The swirl pushed me into the zerba mussel coated bottom and I left a red trail for Jaws for quite some time.
Entry and exit on the East side is quite easy once you are shown the way - public park area for entry and exit with plenty of shoreline parking. The West side exit area was a public lot, but the entry way was more of a permission based (I think....) path off the parkway. Probably tough to locate it without a guide. Definitely worth doing the dive with either shop the first time. I have since done the East side without guides from either shop.
The dive with DipNDive was a paid dive - $30 I believe. The benefit here was a lengthy and informative pre and post dive session. Really helped me out being a newbie and first time in current - my buddy and I were teamed with an instructor. The sessions was conducted very professionally and put me at ease. Well worth the $30!
The Discover Diving dive was (based on my only one experience with them) less formal and free. More of a meeting of divers - all very friendly to dive together. There may be a pre-dive session with them but in my case the group dive was called off due to weather (sporadic lightning moving in) - but many of the geared up divers went on their "own". Given this was a cancelled dive I guess it's not fair to compare.
In short very happy with both groups.
My last dive I found a purse, various bottles old (Labatt Blue stubby) and new, and a chewed up Cubs hat, and a scary looking (potentially toxic filled! ha oh well) steel drum. I discreetly picked up the purse - found no cash but only a very large crayfish hidden away inside. I slung it over my shoulder temporarily for my buddy to see. Saw the explosion of bubbles when he noticed it as he chuckled. Then later in the dive reached down and had it swept (more like yanked) right off my arm by current. Found it quite acrobatic to find a log or rock moving slowly but steadily at you - putting your hand down and cartwheeling over it only to keep moving. Tons of Bass that like to come up and say hi along with small Gobys on the bottom scattering in front of you. SMBs required.
Currents - as they all are - are quite deceiving - again Newbie learning firsthand. There is only one direction to go - and all gear or accessories needs to be stowed or kept track of. This goes for surface and depth of course. Put your fins on in the shallow edge - but don't let go of that strap till they are on snug or say goodbye.
Hope my description was informative. Length due to my desire not to be working now, and instead daydream about diving.