Even the North Sea is far less seasonal than it used to be, think some folks on the forum are trapped in the late 1980’s and early 90’s.
The boats are much bigger and more geared for heavy weather these days. Vessel operators dont wont these expensive assets sitting idle for 3-5 months either. Vessel day rates and who picks up the weather all play a part in winter work. Its not a complete shut down. Typically a construction company such as Subsea 7 will have 2 or perhaps 3 vessels stacked during the winter months across the whole fleet. The rest will be working North Sea or abroad.
North Sea inspection and other non-essential work is hit the most during the UK winter, of course. That still leaves plenty of work UK and worldwide. Its a global industry and see no reason why folks should "expect" to take 3-5 months off.
Not heard anything from the crystal gazers on a slowdown coming up. Agree with the above post that North Sea slowdown is generally Mid jan to Mid March, i’m not splitting hairs buts its nothing like 5 months and hasn’t been for a long time IMHO.