A few shots, all on aps-c:-
18mm Pentax 18-135
Cloudscape at Porthmeor Beach, St. Ives, Cornwall
260mm Pentax 55-300
"Waves against the Walls" at Church Cove, Gunwalloe, on The Lizard Peninsula, Cornwall
300mm Pentax 55-300
Waves at Porthleven, Cornwall
53mm Pentax 18-135
"Afterglow" - Porthmeor Beach, St. Ives
135mm Pentax 18-135
At Dollar Cove, Gunwalloe
There is no perfect focal length, but the zooms give you great flexibility. Personally I'd always take a long - 300mm - lens to get close to the waves/surfers but it all depends on what you want to shoot, the geography of the beach, and what features provide framing for your images.
Plenty more on my Flickr account.