I'm a ham radio operator (45+ years), as well as having a boat. VHF always uses 50 ohm coax. For the 50' run from your radio and up your mast, RG-8X is much easier to work with, and the loss will very acceptable. LMR-400 is much lower loss, but will be VERY stiff and hard to work with. There are some other RG-8U coaxes that have stranded center conductors that would be easier to work with.
Google for a "coax loss calculator" like
http://kv5r.com/ham-radio/coax-loss-calculator/
Here is what I came up with:
2 db loss for the RG-8X isn't bad when you consider that the heavier coaxes can be very difficult to work with. Another thought that would be better would be something like RG-142U, which is a double shielded teflon coated, silver plated coax that has lower loss than the RG8X and is about 1/4" in diameter. It won't age like many other coaxes will. Runs about $1.50/ft.
Mike