Abstract: A body of water having a liquid level and an outer edge is provided with a water heater having a collector formed as an array of adjacent and substantially concentric annular flexible tubes that surround the body at the edge and that has a rectified equal to at least 30 times its horizontal cross-sectional width. The tubes have exposed upper surfaces and feed ends connected to a feed manifold and return ends connected to a return manifold. Respective feed and return conduits are connected to the feed and return manifolds and both open into the body of water below the liquid level thereof. A pump is provided in one of the conduits for drawing water from the body into the feed conduit, then passing it sequentially through the feed conduit, the feed manifold, the tubes, the return manifold, and the return conduit, and for expelling the water into the body from the return conduit.