1991 4.3 cobra hydrolocked after rough run

Hi, took boat out and tubed while on vacation last week. A trim hose had sprung a leak a few days prior so I left drive down just to get kids on tube for a hour or so. Burned through 8 gallons of water in 1 hr. Normal? Trim down cause this? Towed in and refueled. Engine hard start. Will run for 20 seconds or so and die but will die when revved and. Long crank. Operated for 15 minutes off and on Everytime it cranked over good just ran low RPM and rough. Then hydrolocked. My question. Can low RPM rough run allow engine to suck water in through manifold and lock? Again it cranked fine for 20 minutes and then locked. I think the slow idle was due to running out of fuel. I have since pulled plugs and purged water and seems to run ok on trailer. Thanks.

Ayuh,..... Does this motor have the batwing, 1 piece manifolds,..?? If so, I'd sooner they they failed,.....

Low rpms, 'n a rough idle in no way would put water into the cylinders,.....

'n I think yer Right, suckin' the tank dry, pumped a pile of crap into the carb,.....

Runnin' with the trim full down causes the hull to plow, usin' more gas than normal,.....

Had a chance to do some more testing yesterday. Operated engine on muffs for 20-30 minutes in driveway. Didn't like the amount of time between engine start and water exit from exhaust bellows. Also no water through prop. Removed impeller found o ring flat and sucking air. Silicone reseal temporary for testing. Faster prime and fill to block. Engine idles fine. Has big honking cam. Revs ok. Manifolds remain cool through whole test run. Easily hold hands on manifolds. Shut down after 20 minutes and allow to rest. Restart no hydrolock no slow crank. Removed drive to access trim manifold block to replace trim line. Found u joint Bellows water ingress. Torn at 12 o clock. Exhaust bellows charred. Shift cable bellows missing. Gimbal bearing dry and rough. U joints have play. Flapper found in between units in exhaust passage melted to shift linkage and blocking exhaust flow.
I am an auto tech not boat. Mechanically boats seem prettyty simple. Suspect when engine stalled at speed it backwahed water though exhaust due to missing flapper. Will have boat reassembled by next weekend when all parts arrive.