Depending on the brand, it's usually 15% or 20% acid. I dilute it down to about 5% for flushing metal parts (engines, heat exchangers) and circulate it with a drill driven pump.

For your plastic head, using it right from the jug should be OK. But don't expect it to restore an old joker valve. Once those things start to leak back, replacement is the best cure. For the hoses, just take them off the boat and beat them on the dock.