Psalms 69:2

I sink in deep mire, where [there is] no standing
Which signifies not despair of mind, but difficult and distressed circumstances; the Messiah now bearing the filthy sins of his people, and the punishment of them, and so was got into the horrible pit, the mire and clay; (See Gill on Psalms 40:2);

I am come into deep waters, where the floods overflow me:
as afflictions are often compared to waters in Scripture, Christ's sorrows and sufferings are very aptly signified by deep waters and overflowing floods; and therefore rightly called a baptism, as by himself, ( Luke 12:50 ) , when he was as one immersed in and overwhelmed with water.