According to what I read in the "official guide to the tank museum" circa 1980 (when I was lad) - the official cover storey was that the enclosure that the workmen welded to form "Little Willy" (no snickering please - that was 'his' name...) was that it was a water carrier bound for the Arabian Desert...

Wozwasnt is also correct that the cavalry were not interested in tanks even past the start of WW2, at which point they were forced into them- Sir Percy Hobart's biggest complaint against the Cavalry branch was that they were too tied to their Horse's (and playing polo in the case of the 7th Armoured Div in Egypt prior to the outbreak of WW2) to even begin to understand how to use tanks properly.

The nautical link was due to Churchil being the First Sealord for a spell during WW1 and wanting something to break the stalemate on the western front. He carried this through to WW2 where he maintained patronage of his own "private special munitions" group, which ultimately lead to the formation of Hobart's 79th Armoured Div.

Big simplification of history I know - I can suggest/recomend books on the topic if anybody is interested(?)
Mart