Hey Ron, my wife and I will be in Marathon in early May for three days. We've only been there in the fall before, and I wondered what differences there would be at that time of year. We've only collected in Horseshoe after your advice and have had good luck, thanks to you. Pigeon Key sounds interesting since we don't have a boat.
I'd say the thing that will be the biggest difference is that everything seems to be a bit more robust in the spring. It not a huge difference, but I think it's better. That said, the difference year to year is probably a bigger change.
We usually go in mid or late May for the first time, and we've seen HUGE changes from one year to the next. It probably depends more on how the 'winter' weather went. One year, after a very cold snap for just a few days, lots of fish died and the algae bloom was crazy. It covered everything in the shallows were we tend to go and it took months to get better.
Also, being there in May means you are there before the summer afternoon thunderstorms get started. That's a big part of why we go in May. There is very little chance for afternoon storms and no chance for hurricanes. The water is a bit cooler and we take thicker wet suits. But being long term Florida residence, we have thinner blood and prefer the warmer water. Snowbirds think the water is fine even in January... brrrr!
We are also VERY interested in visiting Pigeon Key. We'll probably do some collecting at the Horseshoe, or 2 other spots on the same Key, and then see how Pigeon Key compares. And I'm not sure whether collecting will be allowed there? Or how restrictive they will be in where we will be allowed to snorkel? My hope is they are pretty wide open.
Seeing as how you are from Montana, I'll share a bit of personal agenda. We just got home early this morning (1am) from a week long trip to Montana and Wyoming. We flew into Bozeman and then went to Yellowstone for 5 days. Being 25 year residents of Florida, getting to have fun in the snow was great fun. We watched Old Faithful go off the first morning with all of 15 people out there watching. Two years ago, in the spring, we watched along with how many hundreds of people (maybe thousands)? Did we enjoy it? Well, I took just over 4000 photos!
And not only are you in Montana, but in Kalispell, MT. That's of interest to me because we currently plan to visit Glacier National Park this summer (assuming National Parks are still open given the personnel cuts they are currently facing). Since you are only about 100 miles from there, anything you can tell me that we should visit? We are doing a Road Scholar trip, so most of our stay is very planned. But we intend to arrive a day or two before our tour in order to acclimate to the altitude and visit something (we aren't sure just what yet) that isn't on the Road Scholar's agenda.