tending to float on a liquid or rise in air or gas
Another definition of "buoyant" is "characterized by liveliness and lightheartedness"--this also seems to fit Jim's description of the little boat, because he personifies it by calling it "she" and seemingly comparing it to a willful woman who would never go in the direction she's pointed towards because she prefers to turn around and around in circles.
THE coracle—as I had ample reason to know before I was done with her—was a very safe boat for a person of my height and weight, both buoyant and clever in a seaway; but she was the most cross-grained, lop-sided craft to manage.
Other definitions of "ebb" are "fall away or decline" or "flow back or recede"--these would suggest that an ebb tide should not be dangerous. But an ebb tide is the period between high and low water during which water flows away from the shore, and Jim and his coracle are out at sea.
First she loomed before me like a blot of something yet blacker than darkness, then her spars and hull began to take shape, and the next moment, as it seemed (for, the farther I went, the brisker grew the current of the ebb), I was alongside of her hawser and had laid hold.
Jim uses the word "consort" ironically here. Usually, a consort would be another ship that is deliberately traveling with another ship (as in the consort that Blandly would send out if the Hispaniola doesn't return after five months). But Jim is sneaking up on the schooner in order to take it back from the pirates. A consort could also be the "husband or wife of a reigning monarch" and there is nothing royal about Jim's little boat and situation.
By this time the schooner and her little consort were gliding pretty swiftly through the water;
a narrow channel of the sea joining two larger bodies of water
Another definition of "strait" is "a bad or difficult situation"--this could also fit the example sentence as Jim and his coracle are helplessly "spinning through the narrows for the open sea."
At the end of the straits, I made sure we must fall into some bar of raging breakers, where all my troubles would be ended speedily; and though I could, perhaps, bear to die, I could not bear to look upon my fate as it approached.
A slangy definition of "bail" is "abandon a project or enterprise"--but this would be the opposite of what Jim is doing in the example sentence. When Jim bails, he is removing the seawater that had splashed into his little boat so that he would not have to bail on his chase of the schooner.
Once I shipped a sea so heavy that I had to stop and bail, with my heart fluttering like a bird, but gradually I got into the way of the thing and guided my coracle among the waves, with only now and then a blow upon her bows and a dash of foam in my face.
I have said this was the worst thing possible for me, for helpless as she looked in this situation, with the canvas cracking like cannon and the blocks trundling and banging on the deck, she still continued to run away from me, not only with the speed of the current, but by the whole amount of her leeway, which was naturally great.
the guidance of ships or airplanes from place to place
All told, we had scarce two miles to run; but the navigation was delicate, the entrance to this northern anchorage was not only narrow and shoal, but lay east and west, so that the schooner must be nicely handled to be got in.
In the example sentence, "tremulous" is describing the movement of the water, which is not quivering from weakness or fear. That would be a better description of Jim, who just survived a dagger attack and was looking down at two corpses.
He went in with a sounding plunge; the red cap came off and remained floating on the surface; and as soon as the splash subsided, I could see him and Israel lying side by side, both wavering with the tremulous movement of the water.