Thesen Island is a marina development situated within the Knysna Lagoon on South Africa's Garden Route, in Knysna. The lagoon itself covers approximately 20 km², making it the largest tidal estuary of its kind along the southern coast of South Africa, and the island's internal canal network provides an additional, more sheltered paddling environment distinct from the open lagoon.

The waterways threading through Thesen Island are calm and relatively shallow, making them well suited to kayaks, SUPs, and canoes. A strict "dead slow, no wake" speed policy is enforced on all internal waterways, both for safety and to protect sensitive marine organisms — including the endangered Knysna seahorse and pipefish — that inhabit the gabions and jetties. Motorised craft require a skipper's licence and must display an island identity sticker, but these rules do not apply to human-powered craft. Paddlers should be aware that at low tide it is important to follow channel markers to avoid sandbanks throughout the broader lagoon.

Access is straightforward, with numerous small beaches, mooring spots, and launch ramps distributed around the islands. Kayak hire is available from operators based at the jetty in front of Sirocco Restaurant on Long Street, with single and double kayaks available for one- or two-hour periods between 09h00 and 17h00. Life jackets and paddles are provided with rentals. The SANParks office on Thesen Island is also the place to obtain permits for paddling on certain other nearby water bodies such as Swartvlei.

From the island's waterways, paddlers have direct access to the full Knysna Lagoon, where routes extend past the Knysna Waterfront, Leisure Island, Featherbed Nature Reserve, Brenton-on-Lake, and Belvidere. The Knysna River can be paddled approximately 10 km upstream from the Red Bridge. Those paddling near the Knysna Heads should pay close attention to tidal direction: the outgoing tide creates a strong current at the Heads, and paddlers are advised to be in that area only when the tide is incoming. On windy days, the enclosed channels of Thesen Island offer more shelter than the open lagoon.

Knysna's climate supports year-round paddling, with summer air temperatures reaching the mid-30s°C and winter temperatures hovering around 20°C. Birdlife throughout the lagoon is described as prolific, and the waterways around the island provide opportunities to observe the lagoon's marine life at close range. An annual canoe race through part of the island's waterways is held during the Knysna Oyster Festival.