Or in my case, I had one for the TM bats, and wanted one to bump up the main battery, but needed one for my car/truck too, so I bought regular charger that is mostly hooked up to the boat, but can be used elsewhere. Either way I spent the same amount, I just got more flexibility.
With a battery tender (R) charger, you can do much of the same with some more flexibility, you can get different sets of leads and leave the leads hooked up to the battery/s and just move the charger, which I did for a couple years, problem being Gosh forbid you leave the lights on or something, your car/truck you can’t charge up a car battery enough in a hour or two with a 2amp (big one) battery tender(R). The big unit will set you back about $50, and works fine for a main starting battery on a boat. With all that being said not sure I’d attempt to use it on deep cycle TM batteries, depending on what you read (lots of conflicting bad info out there) they like to be charged up again quickly.