This question strongly reminds me of the series "The Seven Citadels" by Geraldine Harris. Let's try comparing each part of the question.
I went to middle school and high school in Columbus, OH, in the 1990s.
Matches: The books were written in early 1980s. Although they do deal with adult themes (grief, loss, betrayal, jealousy, spirituality) they do so from a youthful (young adult) perspective. I think that they could easily be found in a middle or high school library in the 1990s.
There was a set of fantasy books in the school library. There were four books.
Matches: The Seven Citadels is a fantasy series containing four books:
- The Prince of the Godborn
- The Children of the Wind
- The Dead Kingdom
- The Seventh Gate
The plot (and I don't remember all that much) is that a wizard gets sent on a mission
Near-Match: The protagonist is Kerish-lo-Taan, the third son of the Emperor of Galkis. He is not a wizard, but has the powers of one of the Godborn, which he has inherited since he is descended from the God Zeldin (hence Godborn). The main gift is that he can see through illusions. So although he is not a wizard, it's easy to imagine after 30 years that you remember him as a wizard.
The empire, Galkis, is threatened both by internal threats (the Emperor has withdrawn into passive grief following the death of Kerish's mother, many of the lesser rulers are corrupt or self serving) and external threats (there are multiple other countries that would love to destroy Galkis either for greed or religious obligation).
Kerish is sent by his father, and by the high priest on a mission to find and free the saviour that Zeldin promised. This requires him to obtain seven keys from seven sorcerers. Each sorcerer has one of the titular citadels. They were given the keys (by Zeldin) as a reward for their learning. The keys grant their owners immortal life, so it will be very hard to persuade them or trick them into giving them up. Kerish has no way to force them to give up the keys; nor can they be stolen so his mission seems impossible.
He's youngish, but not a kid.
Matches: Kerish is a young man. He is initially very immature, since he has lived a very pampered life. (The Godborn are so respected that ordinary citizens of the empire are forbidden to even look at them.). During the course of the books Kerish gets wiser.
There is an archipelago, and travel by boat (no, it's not A Wizard of Earthsea), and the environs are kind of tropical.
Near Match: Each book is prefaced by a detailed map of Zindar. As you would expect the citadels are spread out over the entire continent forcing Kerish, and his half-brother (and later a servant, and others) to travel over much of the map. It includes plenty of islands that you remember, and quite a bit of the travel is by boat. But many of the places they visit are on the mainland (around an inland sea).
At the beginning of the second book they visit "Lan Pin Fria" which is a huge swamp through which they must travel by river boat. I think this is the tropical environment. But other parts of the books are in quite different environments (desert, jungle, great plains etc.)
The main character has a guide, a local guy. They get leeches, and the guide cuts them off and eats them, noting that "they are trying to eat me, but I will eat them," or something to that effect.
Exact Match: This happens in Chapter 1 of Book 2.
Dau dived again and within a few minutes they had three intact plants piled in the boat. The Frian [i.e. Dau] scrambled back into the boat and showed Forollkin the trick of getting rid of the leeches that now clung to his hands. He then horrified both the Galkians by eating the leeches he had just detached from his own skin.
"It is not good that they have my blood," he explained cheerfully, "I take it back."
Finally we have
One of the "important people" the protagonist is supposed to meet/find/consult is a wizard, too, and he examines things (plants, animals). They grow large to be examined (by the magic used), but they also die because of it. So this guy's house is surrounded by these giant dead things, like ginormous statues.
Possibly garbled match: The "important people" are the seven sorcerers. The sixth one is called Vethnar. He has set out to use his immortality to study everything, and has filled his citadel with books and with interesting people to talk to. He does create enlarged magical illusions of real things in order to study them.
At end of book 3, chapter 7 we have:
The whole sky was filled with a mass of rich blue and mottled green, crowned with great golden coils. It took him a few seconds to realise that he was looking at a gigantic flower.
But the flower is unharmed. It's just a small flower really, projected onto the sky.
But there are also large natural, but inexplicable creatures that can die if they are gazed upon. Later Vethnar does look at one and it dies. In chapter 9 we have:
Glancing back, Kerish saw that Vethnar had paused beside the great creature whose final agony spread across the slope. He was staring soberly at it, as if he could see the ruined face beneath the flood of golden hair.
I think you combined the near proximity of the creatures enlarged to be studied and the large creature that died by his citadel (but which was not enlarged).
In conclusion we have enough matches that I'm fairly confident that this is the correct book series.
Here is the (UK) cover for the first book, which might jog a memory. Kerish is consistently described as having large purple eyes (a characteristic of the godborn). Of course a US edition of the book might have a different cover.
