I travel backwards and forwards by ferry all the time and it is very strict
We use the ferries a fair bit (I live in France) and tbh I would say it's never been particularly strict before this year. Passport controls have always been pretty cursory until the very recent past. We travelled through Dunkirk a couple of weeks ago and were amazed at how long it took compared to previous occasions (three separate passport checks, with the UK border control check taking over 5 minutes per car).
If the French decide to abandon the Le Touquet agreement I guess the big issue will become whether, and how, tight security at the ports and Eurotunnel is maintained. Will Eurostar and the ferry operators have to shoulder the cost, instead of the UK and French governments?
Before juxtaposed controls, Waterloo (then the Eurostar terminal) was one of the major points of entry for asylum seekers, so I think that regardless of what actions are taken by ferry operators and Eurostar, one would have to expect a dramatic increase in asylum claims at Dover and St Pancras.