Here we have the ‘Old Person of Grange
Whose manners were scroobious and strange
He sailed to St Blubb, in a waterproof tub
That aquatic Old Person of Grange’.

The subject here, is clearly ordained by the text.
The narrative is fairly banal but entertainingly told by Lear in terms of his treatment of the language to tell a simple tale -
In this case - the manner in which the illustration has been drawn graphically - the mood - the content and the composition – and
how the subject is characterised -  is my interpretation
So is the idea of having the Old Person blowing his apron as a sail; for blowing the sail is not in the text.
I thought that the illustration needed to transcend the text a little, without interfering with it.