All sea coasts are at sea level, the clue is in the question. Actually on the beach at any particular point and time, the local sea level is determined by the state of the tide, positions of the moon and sun, prevailing wind, recent rainfall etc, etc which is why maps usually show mean (i.e. average) sea level. Marine charts will show high and low water marks, and depths at low tide. As sea levels rise or fall generally, maps will become out of date. One you come back off the beach, you may drop below sea level - parts of Holland are examples.
For an online map I'd use streetmap.co.uk - the second- highest drill level gives ordnance survey data.