Hello,
As well as Survey Boats, there is lots of ROV work on Construction Vessels, (Crane) Barges and Drilling Rigs.
Equal time on/off 4,5 or 6 weeks is common. Work twice as long as you get at home is also common, as is work anything from 2 days to 3 months whenever they want you to.
You are generally better off trying to work under the rough water.
How often you port call depends on the Vessel and the Work Scope.
Some ships are like a Hotel with single man cabins, your own shower room, and a choice of several relaxing Day Rooms. Some are wear your boots to bed in 8 man bunk rooms, wait to use grotty communal showers, and struggle to find anywhere remotely restful. Food varies from incredible to practically inedible.
Joining in Port, long rides on Taxi Boats and transfer by Helicopter are all common.
Regards,
I5