Yes, the LMR400 has lower attenuation at marine VHF
frequencies (about 1.4db per 100 feet). But, it uses special connectors, usually crimp-on.
RG-213 has an attenuation of about 2.4db per 100ft, or about 1db more than LMR400. However, unless you have a very tall mast you're likely to use less than 100', so the attenuation difference would be less than 1db, i.e., not noticeable. And, RG-213 takes standard PL-259 connectors which, when properly fitted, make a very tight, water
So, I'd use RG213.
use on SSBs, I like RG-214 which has a double-braid which helps to reduce RFI 2 ways (from the transmitter to other electronics
on board, and to the receiver from other electronics