or teach some north sea seals how to crew / hold the cat for you / maybe even a few sea lions to "carry" the cat in and out of the water for you.
hmmm.... design a Carbon Fiber, robotic "spider" that can walk under the trampoline, rise up and connect with the four inside corners of the beams (front and rear), lift the cat and walk it up the beach...
Or, motorized/waterproof beach wheels that you control with remote and can drive the boat up on the beach. All you'd have to do is balance...?
