Guru took to the water alongside island (5)
I believe the answer is:
swami
'guru' is the definition.
(I've seen this before)
'took to the water alongside island' is the wordplay.
'took to the water' becomes 'swam' (past tense of 'swim').
'alongside' means one lot of letters go next to another.
'island' becomes 'i' (geographical abbreviation).
'swam'+'i'='SWAMI'
(Other definitions for swami that I've seen before include "Teacher abroad" , "Hindu religious instructor" , "Hindu mystic" , "Indian religious teacher" , "Religious master" .)