Although it had been a long sea voyage, why do you think Jim said he was not eager to go to ashore on Treasure Island?

From the text, we can infer that Jim was afraid of the unknown, the unknown and the mutinous atmosphere aboard ship. He felt safer onboard.

Perhaps it was this—perhaps it was the look of the island, with its grey, melancholy woods, and wild stone spires, and the surf that we could both see and hear foaming and thundering on the steep beach—at least, although the sun shone bright and hot, and the shore birds were fishing and crying all around us, and you would have thought anyone would have been glad to get to land after being so long at sea, my heart sank, as the saying is, into my boots; and from the first look onward, I hated the very thought of Treasure Island.