Personally I wouldn't take a boat within 5 miles of them - it is bedlam out there and if there is any hint of a wind it can be carnage. It is a totally aerial display so can be seen from a distance.
If you really want to be there, anchor up E Cowes side some hours before and leave some time after to avoid the rush back into Cowes to get to the pub. You will find a significant contingent watching from over Beaulieu side.
The Parade is the best place to watch shoreside, good atmosphere, but again get down early it is restricted to the first 7,800 people! Likewise getting a car in and out of Cowes that night (even if you can find somewhere to park - the traffic wardens are out with tickets until midnight) is a nightmare.
From the Cowes Week website:
The traditional Fireworks display, provided by Cowes Week Sponsorship and operated by the Isle of Wight Council, can be seen from as far as Poole in the West to Chichester in the East. This spectacle is the highlight of Skandia Cowes Week, viewed by tens of thousands of people on the Cowes waterfront, the New Forest Shore opposite and from the hundreds of yachts that moor up in Cowes Roads for the evening.
The Display starts at 9.30pm on Friday 13th August and for a spectacular half hour the sky is filled with the most advanced sequences and stunning colours imaginable. Fireworks are from Kimbolton and sent up from a barge moored just to the northwest of the mouth of the Medina.
For those on the shore, the best vantage points include the eastern end of Castle Hill, Cowes Parade, Princes, Green, East Cowes Esplanade and the grounds of Northwood House.
A superb sound track and commentary adds to the atmosphere and this will be transmitted along the waterfront or tune into Cowes Radio on 106.2FM. Radical Radios will be selling mini-radios for this purpose on the Parade and in the Cowes Yacht Haven.
Please check Island to Mainland ferry times as some crossings are suspended during the period. Please do not discharge any distress flares/rockets within the confines of Cowes Roads and harbour other than in an emergency. You may end up with a £1000 fine. Please note that due to Health & Safety regulations, numbers on the Parade will be limited to 8000.