high water, n. — Green’s Dictionary of Slang

high water n.

[like the real high water, such an economic ‘tide’ will ebb in time, so the image is of impermanence]

financial security.

1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: High water, it is high water with him, he is full of money.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.