I'm going to Minnesota in a couple of weeks and may or may not go scuba diving in one of the 10,000 lakes. What should I expect the water temperature to be if, say, I don't go any deeper than 35-40 feet? Do I need to bring my 7mm farmer john or can I get by with my tropical 2.5mm suit?
I think that ideally you would wear something in between. I switched a couple of weeks ago from the 7mm to a two-piece 3mm freedive suit with a built in hood, and it's about right for me. On the other hand I'm not someone who gets cold easily.
Most of the lakes in Minnesota have a thermocline this time of year with warmish (60s) temps above and colder temps below. In general the best diving is above the thermocline.
At this moment I have no idea which lake I may find myself in. I suppose it varies a lot but do the lakes generally get deeper gradually or do they drop off quickly?
There is considerable individual variation among the lakes and among sites at a particular lake. So, simplifying, perhaps too much, I could characterize several main kinds of lakes:
1) Abandoned mines and gravel pits, now flooded. These offer some of the best diving with something for everyone. The Minnesota School of Diving in Brainerd has developed access points to many of the mine pits in the Crosby area, and if that's where you'll be, you should get in touch with them for information and possibly an organized dive or private DM. Also in this category: ore-b-gone north of Duluth, and many smaller lakes with a local draw, such as Lac Lavon, where I dive regularly. These lakes often have steep walls. Depths vary with several being well beyond recreational limits.
2) Natural lakes in the arrowhead region, including Vermillion and Lake of the Woods. These also have steep drops. I've done a giant step off a dock into 40' of water. Depths vary widely because the topography is so steep, lots of atolls and submerged reefs, deep spots beyond rec limits. The quality of diving is variable depending on your attitude. If I see cool topography, a bunch of big crayfish, a turtle, and a few bluegills, I have just as much fun as on a reef. Especially if I recover a pair of pliers or something. If bright colors are your thing, you'll be bored.
3) Shallower lakes and streams in the fishing region (Grand Rapids, Detroit Lakes, etc). Mostly gradual depth changes, soft bottoms, some have a great deal of aquatic life (plants, fish) in shallower areas. Viz usually poor, not much light at depth, combination of particulates and tannins.
4) Lake Superior, which is in a category by itself. Cold, deep, famously bad weather, best wreck diving in the world. Two major wrecks easily accessible from shore.
The Minnesota DNR "lakefinder" web site has maps that show public access points and depth data compiled via satellite. There are navigational charts available for major bodies of water like the Mississippi, Lake of the Woods, and Lake Superior. There are "state water trail" maps for the smaller streams, which show access points, dams, bridges, and a few other details.
I might be able to help more if you can at least identify the region you'll be in. Minnesota is a big place.
Also can find some info at mnscuba.com. It's been quiet for the last couple of years but the old posts have good info.
There are dive shops in the Minneapolis/St. Paul metro area. Outstate there are very few, Brainerd, Duluth, Rochester, Albert Lea, and I think one or two on the Wisconsin side of Lake Pepin. Almost all are closed on Sundays. So it takes some planning for rentals and air fills.
