Thieves stole sweets and drinks from the cafe at Bourne Outdoor Pool.
Police said burglars forced their way through a pair of wooden shutters at the pool in Coggles Causeway between 8.15pm on Friday and 9.45am on Saturday.
The pool, which opened for its 25th season last month, is charity-run and relies on volunteers to continue.
Police have appealed for witnesses to the theft. If you saw anything, call 101 quoting incident 139 of June 7.