Would this be strong enough for attaching a plastic cable guide to a trolling motor on the bow of a boat that is subject to sea spray and vibration?

Thank you for the question. Unfortunately, there are too many variables in the application described so it is not possible to know what kind of results you can expect. Any success will depend on the composition of the plastic cable guide, how much bonding surface is available, and much stress it may be subjected to regardless of vibration. In situations like this there is no guarantee to product would be suitable.