I suspect the real answer will be Doylist: whatever Jim thinks will make better story.
Having had Harry ambushed by an assassin-squad of winterfae on jetskis, and then a Kraken... having traveled to the island with Denarians, and then an Outsider... he may be "done" with this specific narrative contrivance. Or, he may have more plans for it.
Justine/Nemesis/HWWB, sank the Water Beetle, granted wasn't very deep, but has anyone heard anything about it being recovered/repaired?
Seems like something that could easily be handled (will have been handled) off-screen, between novels. It's likely that it was -- technically speaking -- owned (or at least purchased) by the White Court (not directly by Thomas): remember when Thomas was "out of work" and lived with Harry, he doesn't seem to have independent wealth, he draws upon his family's coffers. Either via salvage or replacement, a boat will be available to Harry if Jim wants it to be!
(although (given that Jim's career is based upon
Making Life Harder for Harry) we may have a few scenes (or novels) where "not having easy access to the island" is a pain in Harry's ass)
If not, will Harry need another ride to the island?
Maybe he'll link up with a Paranetter who's got a WW2-era crop duster, and he'll parachute onto the island. I can just see the opening line of the novel:
One minute ago, I jumped from a plane ten thousand feet above Lake Michigan. The impact was looming with frightening speed.
Or even find an extreme STOL enthusiast... just land in the hilltop clearing beside the lighthouse! See here:
https://www.youtube.com/watch?v=hPakbghLe38Or maybe Lara will just buy her fiance a boat.
Will he find a convenient Way?
...
If the magics of the Castle are similar enough, sympathetic enough, can he create a portal between the two?
Convenient or inconvenient, walking the Ways seems like (if you'll pardon the expression) the way to go (and we haven't explored the Jewel that much, really just in Changes). But we've only ever seen Harry use his "Aparturum," I think? And Harry's narration almost-always describes it as simple & unsophisticated compared to other practitioners who open gates into the Nevernever.
Given the same-y rune-work, I presume some sort of point-to-point "portal" could (in theory) be worked out; but Harry likely doesn't have the magical knowledge: he doesn't understand
how to work with those runes; they're beyond even magic-wonk Bob's theoretical grasp!
I think we first met the point-to-point "portal" in
Changes when Vadderung opens a doorway from Chichen Itza's blood-soaked courtyard to the safety of St. Mary of the Angels in Chicago (an incredibly-dissimilar pairing!), and IIRC Harry had only the very-loosest of theoretical ideas how such a thing might
maybe be done. Much more than salvaging the Water Beetle happening off screen, changing Harry's gatework from "rank n00b" to Vadderung-caliber seems really unjustifiable, narratively; I think Harry could see the advantages, and might decide to go for it, but I think we want to
see our favorite wizard doing that onscreen!
