Lake Ainslie is the largest natural freshwater lake in Nova Scotia, located in Inverness County on Cape Breton Island. The lake spans over 20 kilometres, with a surface area of approximately 57.4 square kilometres, a shoreline length of roughly 48 kilometres, and sits at an elevation of 57 metres. It serves as the headwaters for the Southwest Margaree River, which flows northward to the Gulf of St. Lawrence and is part of the 120-kilometre Margaree-Lake Ainslie system draining a watershed of nearly 120,000 hectares. The entire system was designated a Canadian Heritage River in 1998.

The water often exhibits a jade hue with tea-coloured clarity, occasionally influenced by algal blooms. The lake holds geological, cultural, and recreational significance, with the surrounding landscape featuring forested slopes and farmland reflecting Acadian and Scottish heritage.

Two boat launches serve paddlers: one near Ainslie Village Campground at 1321 Highway 395 on the south end of the lake, and another near MacKinnons Campground. A documented paddling route runs from the south end of the lake to Scotsville, with a return option. From Scotsville, paddlers can continue onto the Southwest Margaree River, which merges with the Northeast Margaree at Margaree Forks before reaching Margaree Harbour on the Gulf of St. Lawrence.

The lake is a primary breeding area for gaspereau and supports a notable brook trout fishery, particularly in spring. White perch are present throughout summer. Bald eagles are a consistent presence over the water year-round. The Southwest Margaree River, originating from the lake, is well regarded for Atlantic salmon and trout, with clear, cold upper reaches providing important spawning habitat.

Ainslie Village Campground, located directly on the lake's south shore, offers facilities including bathrooms, shower stalls, laundry, and a recreation centre, making it a practical base for paddling trips. The campground sits on over 400 acres of grounds.