As can be seen from the sketch map above transit to St Ives from the Grand Union Canal is via the Northampton branch down a flight of 12 locks into the navigable River Nene. Follow this link for navigation notes for this passage available on the IWA website.
From Peterborough one joins East Anglian Waterways and the 'Middle Levels' to navigate to the sluice at Denver to join the River Great Ouse. Follow this link for another description of these waterways. The journey across the Middle Level and up the River Great Ouse to St Ives is relatively simple in comparison to the first part of the journey - it's nearly flat, with only a handful of locks. Ely will be your last chance to visit a supermarket on foot until you get to St Ives but be warned it's at the top of the hill.
There's a nice pub, the Fish and Duck, that does good food at the junction of the River Cam and River Great Ouse with plenty of moorings and other services. It hosts a small marina and boat yard. Another good stop further up river is the Pike and Eel Pub, Hotel and Restaurant at Needingworth/Holywell, It too has good moorings and exceptional food. From the Pike and Eel you're only about 45 minutes cruising from St Ives - lock access permitting.
There are two marinas/boat yards both with engineering and chandlery services in St Ives, one at each end of the town;.
There are two supermarkets in St Ives, Waitrose and Budgen's, located close to the Market Place about 1,000m from the festival moorings with smaller shops much closer. Do your food shopping on Saturday or Sunday because Monday the town centre will be taken over by the Bank Holiday Market. Be warned this is a HUGE event in it's own right on normal Bank Holiday Monday's. This year it is likely to be absolutely packed with the two events running concurrently. (The two supermarkets open from 10 to 4 on Sunday)