Yes! My (now ex-)boyfriend and I stayed at a GREAT B&B that was right on the water in St. Ives.
http://www.cornwall-online.co.uk/carlyon-guesthouse/ - Carlyon Guest House. This place has extremely good rates for its location & amenities. The owners were delightfully nice and you cannot beat the beach view or location!! The rooms are small, but well-decorated with mini kettles, coffee/tea, huge windows for an awesome view, and private bath. You can book even according to which specific room you want, assuming they have vacancy. We had NO complaints about this place when we went during the off season.
St. Ives is a great town. It's big enough to find stuff to do for the day (especially on the weekend, when everything's open for the tourists) and it's got good bus links with the rest of Cornwall. Land's End is within easy reach, as is Penzance. It preserves that small town charm with lots of great restaurants (some right on the pier!) and isn't dirty, crowded, and 'city-feeling' like Penzance.
Good luck with the short-notice booking though. We went in the off season and booked well in advance, so I have no idea how fast this place fills up. Perhaps contemplate planning now for a LATER trip - if you go late season it might be emptier.