trailer bunks

[fla firefighter40]

so i'm headed to the parking lot to get my truck and trailer to load back up my boat after fishing yesterday. one of my bunks are broken right in the middle! i have no idea how that could happen. it's almost like someone purposely broke it. well, i might as well replace them all.
i know i need treated wood, but is there anything else i should add to help with the waterproofing, like some kind of sealer?