most objects like this I have passed in the water seem to be made of some sort of painted steel. I think this is a good combination of durability vs economical. I suspect your electronics will go long before anything corrodes away to nothing, since in my experience its always the electronics that fail first (even when properly sealed for marine use).

Note that the main issue with any barrier coating (assuming there are no collisions) is going to be due to sea life living on/underneath your object and slowly breaking down the material. A copper based metal (read bronze) could be interesting (but expensive) since it tends to stand up fairly well in marine environments and has biocidal properties that prevent fouling/growth. Also very clear to visually inspect since green means good and red means bad which I always thought was very easy to remember.

edit: re-reading your initial comment it looks like you are describing an autonomous vessel of some sort. I used to work at a company that produced such devices and they were made out of composite plastics (which have their own issues that cause them to break down eventually in the water as well).