ferry dover to calais, tips and experiences please!

[LilacBump]

we're going over to belgium in february and we will be driving down to dover and take the ferry (P&O) to calais. i'd appreciate some advice and experiences. will it be best to book beforehand?

Yes and shop around, prices vary greatly.

Don't just look at P&O (unless you have shares or whatever and can get a cheap ticket). The last few times I have been I have found SeaFrance the cheapest (thry also have childrens soft play area & good food). Also look at Hoverspeed (but could be a proe to bad seas in Feb!), Speedferries and also Norfolk Lines (Dover-Dunkirk).

Also dont do an overnight trip if you have kids. Its cold, they turn the heating off and its every man for himself as far as seating goes.

We used Speedferries after Janh's recommendations and they were fab. But...they sail to Boulogne, which is a bit further West than Calais, and as it is a SeaCat it could be prone to bad weather cancellation/delay, as lapsed runner says.
Sadly Hoverspeed have just gone out of business so it is down to P & O, SeaFrance and Norfolk Lines.
LilacBump, if you are travelling in half-term then I would recommend booking, even in Feb the ferries will be more booked up.
Is there a reason why you are not looking at Eurotunnel? It is a joyless experience going through but the terminus facilities are nicer than Dover Ferryport nowadays IMO and it is less hassle with children unless they are old enough to behave as asked and enjoy the ferry...(we have one of each and could cheerfully have tossed dd overboard last time whereas ds loves boat crossings now).

If the weather is just barely good enough for a hovercraft, it will be very very unpleasant. Something like half the people threw up, the one time I was on one. Lots of bumping up and down + small windows - any fresh air = lots of vomit.