For a number, boarding the boat is the high point (8)

I believe the answer is:

pinnacle

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'high point' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for pinnacle that I've seen before include "The highest point, summit" , "Crown" , "Highest point (of fame or building)" , "The highest point, peak" , "The highest level" .)

I've seen this clue in the Evening Standard.

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