I give my 'general' tools a light spray of WD40 and keep them in a box designed for heavy machine-gun ammunition. It has a waterproof seal. The ones for electrical work live in a plastic box.
I think your mistake is probably using a tool bag rather than a box. Its bound to get the odd splash of seawater on it and will, as a result, be forever damp.
I use a small plastics tool box. Most of the tools have been in there for over 30 years without going rusty.