The board is waterproof. The joints are not, until you seal them.
wedi boards have an XPS foam core, so water does not pass through the sheet the way it can through cement board, which is why UK installers ask whether they still need a tanking membrane over the face.
You still seal every wet-area joint, corner and penetration with a bead of wedi 610 plus joint reinforcement tape, or wedi 520 with sealing tape, and then you tile onto that. If you want the fitting order in one place, see how to install wedi boards.
When extra tanking is specified
If the spec calls for an extra membrane, a different wedi board such as Vapor (which uses wedi 620 and sealing tape), or a detail the standard board system does not cover, follow that spec and still seal the joints. If the datasheet and the spec disagree, raise it before you order.
What BBA 00/3675 covers
BBA Agrément Certificate 00/3675 (Issue 4, 31 July 2024) assessed wedi building boards as an intermediate substrate for ceramic and stone tiling on internal walls and floors. Read the live certificate for limitations. Confirm the current status with BBA before you rely on it.
Shop boards and 610
You will want the boards plus 610 or 520 and tape. The datasheet is on each product page if you need to check a detail before you order.