the problem is range and lethality.
swarm boats would have small guided missiles and rockets that have a range of 5+ kilometers and some of them are suicide boats. these boats may be armored to an extent.
a m-240 has a range of 800 meters at the maximum above that you cant expect to hit and destroy something reliably…

Having the tank 100% electric won't work but what will is having it with a hybrid electric motor with a generator to generate power. They could do this right now with the current tanks, all they need to do is replace the huge gear box with an electric motor and the massive batteries with lithium batteries. One problem…

the cost of shipping would be more then you would get from the metal