Skin fittings on boats may often be close to the waterline or even below the waterline . I don;t have a picture of skin fittings below the waterline but I do have three on my boat and there is nothing unusual about that.
What is important is what happens to the hose attached to the skin fittings inside the boat. The hose or the item inside the boat has to extend sufficiently far above the waterline to prevent sinking .
On a lumpy water boat this has to consider the boat heeled over as in when sailing, when a powerful motorboat is in a turn, or the effects of waves .
On a canal boat somewhat less of a risk, perhaps but still there can be waves even if on a modest scale.