It's not illegal per say to spread ashes on the beach in Florida, although spreading ashes on the beach in Florida is regulated, mainly by federal law.
Federal law requires that ashes on Florida beaches must be scattered at least 3 nautical miles from the shore.
So if you spread ashes in the water in Florida beaches it must be done 3 or more nautical miles from the shore.
If spreading the ashes on the beach itself in the sand the ashes must be dispersed and spread out and not just dumped one location.
In Florida for any national parks and federal land you must get a special park use permit from the national park service before spreading of human ashes.
My grandmas ashes were spread on a beach in Florida where she wanted her ashes spread.
I didn't ask permission but I was told it was not legal and so I just placed her ashes in a Ziploc bag and discretely spread them out and nobody said anything to me.
And if you want to scatter ashes in inland waters like lakes or rivers you may need to contact the state's Department of Environmental Protection or the local authorities for their guidelines.