Sailor's hat boy and I found in lake (9)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

PAUL (boy) + I contained by [found in] TARN (mountain lake). I haven’t checked the whole archive but TARPAULIN as a hat has come up at least once previously, in a 15×15 in December 2019. On that occasion it was defined simply as ‘old hat’ but the clue used ‘sailor’ in the wordplay to provide TAR. I didn’t know it then either but as with today’s clue the answer was easy to construct.