Corrosion Tendency Between Galvanized and Stainless Steel

There's about 0.4v difference between stainless & zinc in the galvanic series so there is a real risk of corrosion happening in water.

You could have a major problem unless you can isolate the two metals with a non reactive product such as silicon, rubber etc. However you must be careful that the product you use can withstand the underwater environment and should request specialist advice in this area.

It is common practice to weld stainless steel to mild steel using 309L electrodes, but under water you would have a problem.

It depends upon several factors; conductivity of the water, type of SST and whether it is active or passive. For example, active 18-8 austenitic SST is very close to steel in the galvanic series whereas passive 18-8 is not. The galvanizing (zinc) is anodic to both steel and SST and, in fact zinc, magnesium and commercially pure aluminum are frequently employed as sacrificial anodes for corrosion protection of underwater structures (outboard motors, ship hulls, hot water tanks, etc.). For critical structures, cathodic protection systems are often used to negate galvanic corrosion. In that case, an opposing emf from an external source is applied to the structure being protective to ensure that it is cathodic to its surroundings.

Thanks to all who responded to my question. All of the responses have been very helpful.