Coolant- What To Use?

Water, but give it some additive- it'll prolong your water pump life and has corrosion inhibitors to keep your water jackets from flaking and caking.
Many out there, 2 examples:
BALDWIN FILTERS Coolant Additive: Coolant Additive - 4ZMU1|CS5007 - Grainger
Royal Purple Radiator Additive 01600 (xtremediesel.com)

Zinc anodes (sacrificial) help keep the dissimilar metals exposed to your cooling system from reacting to each other, resulting in corrosion, pressure leakages and ultimately failed parts. The marine industry commonly uses this method to protect hulls and engines from seawater corrosion.
They commonly come in threaded rods that you cut the length to fit in an unused manifold heater hose fitting or similar port. Replace them every season.
ANODE 194-CME0 SIZE 3/8"X1-3/4" PLUG 1/4" PIPE UNC5/16" PENCIL ZINC ENGINE BOAT | eBay