Ferry from Caen to Portsmouth

Clearly in October there is the chance of severe weather. The ships are good ships, and if they sail chances are that you will have a good sailing, unless you are prone to mal de mer. The passenger experience is very gallic.
As the month wears on the chance of weather disruptions increases.
The weather cannot be predicted (see this weeks storm in Holland and storms for tomorrow due in Scotland).
If the weather is bad enough yes the sailing will be cancelled. Not usually due to the ship being unable to weather the storm, but more because of windeage issues when undocking and docking.
It happens.
At Caen you have to catch a bus from Caen station to the port at Ouistreham, and at Portsmouth you will need a taxi for the c 1 1/2 miles from the port to central Portsmouth.
The 0830 has no bus connection at Caen- that is taxi to the port. The 1630 has a bus connection.
If you require a guaranteed passage then you have to return via Eurostar.

Though I have not taken the Caen-Portsmouth route, I have taken a ferry for roughly the same distance and same waters. In July 2010, I took a ferry from Cork, Ireland, to Swansea, Wales.

The weather and seas were fine. Zero hassle. No cancellation, though the boat was maybe 40% full. It had a cabin for the trip that left at 8 pm and arrived at 6 am.

Thank you very much.

I've never done this route, but I did St Malo - Portsmouth and the Dover/Folkestone - Boulogne/Calais ferries quite a few times back in the day. I have encountered rough seas in August and in December. They can happen any time in the English Channel. Complete cancellation seems to me highly unlikely but always possible due to strikes. Hassle factor: typically with ferries around the British Isles you end up having to carry your luggage a fair way, often up/down stairs. The Eurostar is a dream in comparison. But the ferry can be pretty fun in its way.

I have always wanted to travel this way between countries. Unfortunately, the one time I tried, I was going to go between Poole, UK and Cherbourg. When I arrived at the ferry terminal in Poole, they informed me there was a strike on the French side and my ferry was cancelled. This was in the late 90s before wide-spread cell phone usage (i.e., I didn't have one there). To their credit, the ferry company put me in a taxi at their expense to Portsmouth, where we took a ferry to Ouistreham—which is where the actual "Caen" ferry arrives. It worked out in the end, but it made me wary of trusting my travel plans to this ferry route.

I’ve done Cherbourg-Portsmouth, and Calais-Dover at least 4 times. Twice it’s smooth and easy and twice it was so bad i was horribly sea sick the whole time. Now i much prefer taking the Eurostar. Quicker and easier.

I've scheduled five ferry trip over the years between St. Malo, the Channel Islands and Poole or another English port. All the trips fell between late May and early September. Two of the five sailings were canceled, one of them for three days in a row. Unless you have a loose itinerary and can be flexible--and possibly pay for two hotel rooms on the scheduled travel day)--I view those ferries as somewhat risky.