316 stainless steel will rust slightly in sea water, and a small amount of corrosion will probably be clearly visible. The Vectrino and Vectrino Profiler were designed as freshwater lab instruments, and longer-term sea-water deployments are not recommended.
If you have already deployed the instrument in coastal water and observe a small amount of rust when performing a visual inspection after the deployment, you should rinse the instrument properly in fresh water. Note that a bit of rust should be of no consequence for a reasonable number of (short) deployments, as long as you rinse it afterwards. However, remember that it is a lab instrument, and the probe will not have an indefinite lifetime in sea water.