Don't forget to wear earplugs. Getting a barnacle critter swimming into your ear is an extremely painful experience. It's also quite difficult to get it out!
Were these MAX-N charts? Lowrance HDS Gen 2 plotters are compatible with C-Map MAX-N and Gen 3 units with MAX-N+. They will not work with C-Map MAX or C-Map 4D.
See here.
I'd go with Guidi "Non Stick" Bronze Seacock Full Flow Valves Type "ALEX". They are a type of Plug valve, but the plug doesn't turn like a blake's seacock, it rises up and down similar to a gate valve. Importantly it cannot be overtightened. The plug is made of neoprene.