Radak

I am weary with my groaning; Every night make I my bed to swim; I melt my couch with my tears. – For I groan and am anxious on account of my sickness and weep until I make my bed swim every night, because at night the sickness is grievous, and a man laments and weeps on account of his sickness. Or he weeps at night because the household is asleep and no one sees him. And lo! he is weary with groaning and with weeping. And as for what he says: I make to swim and I water – it is by way of exaggeration and hyperbole. And the interpretation of אשחה (I make to swim) is (to be derived) from (such passages) as: “as he that swimmeth (השחה) spreadeth out his hands to swim (לשׂחות)” (Is. 25:11); or from the Targum (rendering) of “and he washed (וירחץ)” (Gen. 43:31), which is ואסתי. And the interpretation of אמסה (I melt) is (to be gathered) from (such passages) as: “they made the heart of the people melt (המסיו)” (Josh. 14:8); “and like a moth Thou makest his beauty to melt away (ותמס)” (Ps. 39:12). These (instances) are from verbs lamed he of the hifil conjugation. And there are many (cases) of double 'ayin verbs of this signification, as: “They have made my heart to melt (הֵמַסּוּ)” (Deut. 1:28); “and the heart of the people melted (וַיִּמָּס)” (Josh. 7:8).