Friend lost a 24" Diameter prop coming out a canal after he just bought a boat shaft snapped in 10ft of water and never found it. You will be lucky to retrieve it.

Suction cups. Diver that cleans my hull uses them

I am in buffalo ny. I have a friend that dives. He found my cousins engagement ring after a week or so in the waters of Lake Erie.

It all depends on how accurately you can go back to where you lost it.

Padraig.

https://www.kjmagnetics.com/blog.asp?p=magnet-fishing

What’s magnet fishing?

At left, an MMS-C magnet screwed onto an MMS-D magnet, with the rope tied in between. At right, a single MMS-G magnet.
Magnet fishing uses a strong magnet to try and grab ferrous metal objects that are hidden or lost underwater. If you want to try grabbing something lost on the bottom of a lake, pond, or river, a magnet on a rope might do the trick.

What magnets should be used? As mentioned in our article about Mounting Magnets, many of our customers have reported success using strong Mounting Magnets. They are durable, and you can easily tie them onto the end of a rope.

Just depends on whether you have a relative location...15 ft and muck is nothing to worry about...As someone stated earlier, you need to maintain a grid and use a definitive search pattern...I too am a search/rescue/salvage diver for Unlimited hydro, GP's, F2's and OPA Offshore..I find props and shafts all the time, with a relative position and search pattern...In fact, I, along with our dive master, found the Ellstrom skid fin, which was lost during a blow-over and crash near the end of the 2015 racing season...They needed it for the San Diego race...So, we dove for it a week after it happened, with only pictures of the hydro flipping from various angles as a reference...and after almost 4 hrs. of searching in the water (overcast, windy, low visibility and against the current)...We found it, towed it in and shipped it to the Elstrom team...Though, they were unable to make the repairs in time for San Deigo...So, in conclusion, your challenge is not to find a diver...but, to find a knowledgeable one, that's not afraid of Blackwater, seaweed, low visibility and/or current...And if you don't find it, there is always next season...and you can look for it then...it isn't going anywhere...Note: I may be able to help out, if you don't find it...I would post pix of I could...

Im almost afraid to ask what else you found on the bottom of mission bay....

I'm experienced in doing this. Not my first rodeo.

Get a diver in asap..
I dropped an anchor in 10' of water once. Jumped in and found it. That's about as deep as I can dive un-assisted before I lose breath and my head explodes. Found the anchor on the fourth dive down, brought it to the surface and frantically treading water, my non nautical friends didn't do much to help me lift it into the boat. Damn those things weigh you down!

Also had a golf club returned once after it got thrown into a lake. Having a drink after golf and my caddy came to me with the nasty 7 iron and said "I get club for you mister, I swim and find". Gave him a very nice tip.

Things can come back if someone makes the effort.

RR

You will need them