To get local information the best option would be to swim at a lifeguarded beach, as safety flags will be flown according to the conditions as the lifeguards see them.
http://www.rother.gov.uk/beaches
https://rnli.org/safety/beach-safety/beach-safety-advice
https://coastguardsafety.campaign.gov.uk/
Besides the above, the most important information is what the tide and weather is doing. Below are two other useful links.