what experience does ishmael have that provides him with sensual pleasure? what does he have visions about that night?

The crew of the Pequod squeezes the sperm from the whale caught by Stubb. Ishmael feels an "abounding, affectionate, friendly, loving feeling" at this avocation, and wishes that he could "keep squeezing that sperm forever." Ishmael then describes the particular process of preparing the whale sperm for the try-works. To develop any definitive conclusion about Ishmael's sexuality from this chapter is nearly impossible and amounts to severe revisionism, yet Melville builds a strong foundation for innuendo and conjecture without building it into anything concrete.

He finds squeezing the lumps out of the whale's sperm to be sensual;

"I found it strangely concreted into lumps, here and there rolling about in the liquid part. It was our business to squeeze these lumps back into fluid. A sweet and unctuous duty! No wonder that in old times sperm was such a favorite cosmetic. Such a clearer! such a sweetener! such a softener; such a delicious mollifier! After having my hands in it for only a few minutes, my fingers felt like eels, and began, as it were, to serpentine and spiralize."

Ishmael's dream;

"But that night, in particular, a strange (and ever since inexplicable) thing occurred to me. Starting from a brief standing sleep, I was horribly conscious of something fatally wrong. The jaw-bone tiller smote my side, which leaned against it; in my ears was the low hum of sails, just beginning to shake in the wind; I thought my eyes were open; I was half conscious of putting my fingers to the lids and mechanically stretching them still further apart. But, spite of all this, I could see no compass before me to steer by; though it seemed but a minute since I had been watching the card, by the steady binnacle lamp illuminating it. Nothing seemed before me but a jet gloom, now and then made ghastly by flashes of redness. Uppermost was the impression, that whatever swift, rushing thing I stood on was not so much bound to any haven ahead as rushing from all havens astern. A stark, bewildered feeling, as of death, came over me. Convulsively my hands grasped the tiller, but with the crazy conceit that the tiller was, somehow, in some enchanted way, inverted. My God! what is the matter with me? thought I. Lo! in my brief sleep I had turned myself about, and was fronting the ship's stern, with my back to her prow and the compass. In an instant I faced back, just in time to prevent the vessel from flying up into the wind, and very probably capsizing her. How glad and how grateful the relief from this unnatural hallucination of the night, and the fatal contingency of being brought by the lee!"

This is found in Chapter 96