That'll be the grommet closed up where the water pipe meets the adaptor plate. It's a common problem, we've done 2 this week. Mercury have a re work kit to solve it!
You'll need a 32-897971A01 water tube/seal kit, 27-892320001 adapter plate gasket, 27-892319001 powerhead gasket, 10-4007455 bolt, as it usually snaps when you remove it. £164.45 incl vat!!!!!!
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