Short answer
Draw a sample from the drive's designated point and inspect it against clean oil. Water shows as a milky or grey emulsion, a separated layer at the bottom, or a level that has risen. Any of those means a seal has failed and the drive needs attention before further use.
You will need: Sample container, torch, correct drive lubricant, replacement sealing washers
Saildrive oil is a diagnostic sample as much as a lubricant. Checking it regularly catches seal failures while they are cheap.
What you are looking for
- Milky or grey emulsion: water is mixing with the oil. A seal has failed.
- A separate layer at the bottom of the sample: water that has settled out.
- A rising oil level: something is getting in.
- Metal glitter: internal wear, and a different and more serious conversation.
Compare against fresh oil of the same grade in the same light. Subtle contamination is easy to miss against a dark background.
If you find water
Stop using the drive and get it assessed. Water in a saildrive comes from a failed seal, and running it accelerates the damage to bearings and gears. This is not a top-up-and-watch situation.
While you are there
The saildrive's other maintenance items are on the same visit: anodes, the condition of the protective coating, and the age and history of the hull diaphragm — a safety-critical, time-limited component whose interval is set by your specific drive maker and must never be copied from another maker's schedule. See saildrive maintenance and corrosion and is a saildrive less reliable than a shaft.
Coatings and antifouling
Copper-bearing antifouling applied directly to an aluminium leg promotes serious corrosion. Use only coatings approved for the drive material — and in brackish water, check anode alloy choice as well.