The first thing to grasp is that the ferry companies (Wightlink and Red Funnel) operate their car ferries much like a budget airline or a hotel chain like Travelodge. That is, the prices go up as the ferry places fills up.
So, the earlier you book your Wightlink or Red Funnel ferry, the cheaper it will be. Similarly, if you can catch a ferry at an unpopular time then you will pay less. Saturday lunchtime in the school summer holidays is obviously more popular then 11pm on a Wednesday in January. Admittedly that’s not much help if your self catering changeover day is Saturday.
I haven’t quite worked out if Wightlink and Red Funnel release their new batches of ferry tickets routinely or not (train companies release tickets three months ahead). If anyone knows the answer, please get in touch.
The same pricing structure doesn’t really apply to Isle of Wight ferry foot passengers on Wightlink, Red Funnel and Hovertravel (Hovertravel only do foot passenger tickets). The prices fluctuate a bit based on time of day, but generally you can just turn up and pay the same in August as you will in January.
The next thing to consider is whether there are any offers or discounts on Isle of Wight ferries which you can exploit. My favourites are discount codes and promo vouchers which turn up fairly regularly (we keep these up to date on our ferries guide) and the Wightlink deal with Tesco Clubcard or for disabled blue badge holders.
There are other loopholes and deals added regularly to our ferries guide, such as Wightlink’s deal with Camping and Caravan Club Members, but I won't start listing them all here.
Another key point is that the car ferries charge per car rather than per person. It might not be practical to share transport with another family if you are staying for a week, but it is worth considering for a short break or if you are travelling for a festival (Isle of Wight Festival, Cowes Week, Bestival etc). If you are travelling alone and won't use the car much then it will be cheaper to come as a foot passenger.
If you are booking accommodation then there is sometimes a bundle deal to be had. For example, Park Resorts often give free off peak ferry travel to their four Isle of Wight caravan parks, whilst Warner Leisure Hotels and Blue Chip currently offer a discount. Some smaller independent hotels and guests houses also get special rates.
There are other tips, tricks and loopholes in our more comprehensive guide to discount Isle of Wight ferry travel.