They are constructed differently for a reason. The marine pump has an extra diaphragm to keep fuel from dumping into the bilge when the first one fails. An auto one doesn't so the gas/fumes have a chance to flow out into the bilge and ignite and blow up your boat and kill everyone on board. When a marine pump fails, the gas is siphoned up through the carb and floods the engine so you can't continue to run it like that.
This is a serious life and safety issue that no amount of cost saving is worth.
I am new to boating and have the same problem finding a replacement and don't want to create a bomb..... I purchased my first boat 5 weeks ago so still learning alot, the previous owner installed a electric pump to bypass the failed mechanical one.
My engine is a OMC 302amftc model with a Carter pump M60032.
Here is a pump claimed to be a marine pump but has a weep hole where I assume gas would come out in case of a diaphragm failure.

My current pump has a fitting instead of a weep hole which had a line attached at one time. I assume this line would go back to the carb as a saftey in cause of a failure to prevent filling the engine compartment with fuel. Any suggestions apprecaited.