The RNLI says the man and boy were were swept out to sea off Aberffraw, on the west coast of Anglesey.
The man was able to make his way back to shore, but the child is still missing.
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Mulder had to wait 408 days for his Swans bow last week and was their saviour at St Andrew's as the visitors claimed a 0-0 draw on Friday.
Missed Friday's Wales at Six? Catch up here.
The rain will clear up this evening, leaving a fairly quiet night to come with some pretty mild temperatures overnight.