A watch system, watch schedule, or watch bill is a method of assigning regular periods of work duty aboard ships and some other areas of employment
https://military.wikia.org/wiki/Watch_system

Watchkeeping or watchstanding is the assignment of sailors to specific roles on a ship to operate it continuously.
https://en.wikipedia.org/wiki/Watchkeeping

--------------------------------------------------
Note added at 14 mins (2021-03-10 03:49:43 GMT)
--------------------------------------------------

STANDING ORDERS - Yacht Captain Worldwideyachtcaptain.co.nz › StandingOrders
PDF
Crew on Watch will follow the weekend Watch routine as outlined. The Watch keeper is to be in uniform and to carry the Watch keepers mobile with them at all ...

>>>

--------------------------------------------------
Note added at 14 mins (2021-03-10 03:49:51 GMT)
--------------------------------------------------

This is the most generic term.

Look closely at the information we have. It's awkward to add this post, but I need space to explain what I'm getting at. Sorry if those having already posted consider that I am not adding anything new. I do realise that the expression I suggest includes a term already suggested. Just bear with me. Do not bear away (deliberate pun).

"Tout navire amarré ou mouillé dans un port doit avoir un service de garde.
Dans le cas où le service de garde serait jugé insuffisant pour empêcher toute dégradation aux installations du port ou aux autres navires, les officiers de port peuvent le faire compléter suivant l'importance du navire."

Apart from the standard COLREG/RIPAM rules that include provisions relating to watch requirements when underway in order to avoid/reduce the risk of collision, other watch requirements will arise in specific circumstances. The example we have here clearly sets out the situation:
- requirement for the ship to keep watch ("ship" also reads to mean the crew)
- when berthed or moored/at anchor within the harbour/port
- reason: to prevent/reduce risk of causing damage to port installations/facilities and/or other ships in the vicinity.

Therefore the ship must ensure that their watch comes up to scratch requirements of the harbour authorities. If the ship's ability to keep watch, if the ship's watch capabilities are considered to be insufficient, then the harbour/port authority will provide an extra bod or two.

The use of "service de garde" strikes me as a definition that is a little open to interpretation. I think that is an obvious choice for the harbour/port authority here. They wish to have the final word as to whether the on-board watch are sufficient. The sufficiency may be related to numbers available to do the job but it may also be connected to organisational issues or even, why not, how much the crew drink now they are finally not underway. The point is that those keeping an eye out have to meet the standard expected by the local authorities.

Bearing that in mind, it still has to make sense and com within the nautical and maritime way of phrasing this. "Watch" has to come into it in my view. This is the specific term I see used whether a vessel is underway, berthed or moored.

Authentic English ways to express this are more likely to be based on a verbal expression than a one-word match for the French. I think the French notion of "service" here can be read as meaning ensuring that someone (sing/pl) does keep watch and that a large vessel ("importance" here being to do with size, not whether ERII is onboard or not, for example) may need more people on the task than they are able to provide, or that they are poorly organised. Then the harbour authority can step in. So yeah, "to keep watch" covers that.