would i
anyway how about a one ton builders sack with 3 water butts {i know you can fit 3 in a sack} in it with the lifting straps of the sack tied together {to stop the butts escaping} fill all with water winch the sack down to the cannon using weight of the cannon as the dead weight then when on the bottom {well its only 15m!!!} fill butts with air
or use same thing but with all the stoppers in the butts with air in them put a few ropes around the cannon and then ropes from the butts threw ropes on the cannon and then back on the boat
boat moves away
cannon should move and come up! then you could get it into even shallower water
{ive used this with a big anker we found!}
they do the same thing as a builders sack but for potatoes and are much better they have lots of holes in them just like a 1 ton onion sack so the water cant get trapped inside the sack {i have a few i used for lifting plates off the duke!

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i want a cannon

need any help
i have lots of lift bags as well