I’m PADI certified but only dive on vacations. I have a mask, snorkel, and fins.

I’m interested in a small pony setup or something that I can use on Lake St Clair to help me clean my boat. I regularly take my boat out to the sandbars where the water is only about 4 feet. I’d like a small air tank to be able to clean the hull bottom easily.

Are there any recommendations for a small tank and reg? I don’t need a BC or weights for what I’m going to do.

I used a 19 cu foot pony for that...any lower cost first and second stage will do for that and they make a small no hose pressure gauge

Do you have any links? I don’t even know what sites are good for equipment. I might just go to a local scuba shop.

not a bad idea to go local as you still need a visual inspection. You can get a low cost regulator from leisure pro but I think all their equipment is gray market. If you want it would make sense for you to buy a high quality regulator and use that on the pony and when you go diving....kind of a two for one plus you know exactly what you are using as opposed to a rental. In that case you would also need a regular pressure gauge/computer set up...and BC hose...anyway that would be a better way to go...I use a sealed Atomics B2

How big is your boat?

I spent a lot of time under mine...you may need more than you think.

Skip the tank, get a Hookah system.

Years ago one of my dive buddies had a dual regulator Hookah system.  We used to earn extra money cleaning boats in the Chicago marinas.  You would be surprised how anal sailboat owners can be about keeping the bottom of their boats clean.  We also collected golf balls from the water hazards at several of the local country clubs.  The pro-shops sold them as "practice" balls.

It’s 35 feet. I don’t need to clean the entire bottom though. I have bottom paint. I need to clean the waterline and the sterndrives.

You can get a lot done with a snorkel.  I used a snorkel for the waterline area and a al40 for the rest.  I had a Cal 27, in salt water that was a monthly project.  It drew 4.5', displaced 6,400 lbs
I would not get under a boat (you mentioned you didn't plan to) in no less than 10'.
That's a big moving hazard above you.  After a couple of bumps to my melon, I started using a Petz helmet that I had.  Of course being deeper water, you will need to run a line under the boat so you have something hold on to/against so you can scrub.
I think a 19 plus a snorkel would get it done.  Going a little bigger wouldn't hurt. My consumption avg is .41 - .47.  Rate went up considerably when cleaning.
Carrying it is another issue.
I have an old Seaquest jacket bcd that I removed the tank strap (std won't work) and used two hose clamps to secure the smaller diameter tank.  Worked well.
Going back to the snorkel; I was able to drop the spade rudder (4') and reinstall in water with just a snorkel.  The al 40 was handy for the deeper cleaning, zinc changes, and screw swaps.

Man, I am glad I got rid of that boat...

Get a 60ft hose and float the tank.