The River Thames is of course the reason for the existence of the Castle and hence the town of Windsor. The river is probably the best place to view the Castle, and from here it is possible to appreciate its strategic site. Best views of the Castle are from the Eton bank (cross the bridge and walk a few hundred paces into the open meadows for the clearest view).
If you're looking for more excitement, try hiring a self-drive motor boat or rowing boat from Kris Cruisers or Gamble & Logie Motorboats by the riverside. You can also take river cruises upstream to Maidenhead or downstream to Hampton Court with French Brothers. A favourite activity for younger visitors and residents is feeding the swans.
If you have your own boat, its possible to moor here overnight.
See this document all about boatyards and marinas: PDF FILE
Cyclists and walkers can travel along the Thames Path both upstream and downstream from Windsor.
Also see where to stay in Windsor