DUMPING LITTER PROHIBITED.—Unless otherwise authorized by law or permit, it is unlawful for any person to dump litter in any manner or amount:
(a) In or on any public highway, road, street, alley, or thoroughfare, including any portion of the right-of-way thereof, or any other public lands, except in containers or areas lawfully provided therefor. When any litter is thrown or discarded from a motor vehicle, the operator or owner of the motor vehicle, or both, shall be deemed in violation of this section;
(b) In or on any
freshwater lake, river, canal, or stream or tidal or coastal water of the state, including canals.
When any litter is thrown or discarded from a boat, the operator or owner of the boat, or both, shall be deemed in violation of this section;
This is Florida state law as it apply to Monroe county and other counties as well.
From a legal standpoint dumping the tank in Florida water is not legal. The fact that it could be or may not be actually harmful has nothing to do with the fact that it is illegal.
Unless otherwise authorized by law or permit you cannot dump stuff in the water in Florida state waters.
Now...what actually goes on around Marathon is that a few people take the risk on dumping metal junk in the water for the purpose of creating small artificial reefs for fishing. The fish seem to be very happy but getting caught doing this will most likely land you in jail. You can find all the info in detail here
http://www.leg.state.fl.us
I assume others federal laws regulate dumping stuff out to 200 miles. What they are and what you can and cannot do I don't fully know.
I personally don't dump stuff in the water because I have a vested interests in the ocean and to some extent want to obeying the law. I will fish on the junk other people dump...lol
As far as the law in general goes, we use to have something in the law called "the reasonable man" Problem is he died in 1956 and had no children.