Had the same problem with my 1988 after a few years of ownership. Iboats probably sells the water jacket gasket. Being a '84 model and if used in salt water, you may be referring to water inside the cylinder, not water squirting out, visible from outside the engine, while running, from around the spark plug. The gasket replacement is for the latter....water visible from outside the engine.
Your biggest problem in replacing it is getting the SS bolts out without breaking off which is usually the case. I think it was Racerone that posted, years ago, that he grinds the heads off so that he can remove the water jacket cover. Once removed, he soaks the threads with a quality penetrating oil and then removes the bolts with vise-grip pliers. Sounds like a plan as the break is usually right at the block surface because from the outside, (with the water jacket cover still installed) the penetrating oil can't get down into the thread area.
Bolts are SS and "Fastners" company stocks all types if Iboats doesn't have them or your local dealer or NAPA can't get them for you. Run a tap in the threads to "chase" them....clean out the corrosion and I liked to put Blue Locktite (thread sealant) on the bolts prior to installation.....lubricates the installation for better torque setting, locks the bolts from vibrating loose, and helps to prevent further corrosion.