Here the speaker imagines a strange wish: if light were a word, they could speak it, and their tongue would turn into a glass-bottom boat. Those boats let people look down through the water at what lives below without disturbing it. So the speaker's voice becomes a way of seeing as well as saying.
The "you" in the poem is the water, deep and mostly hidden. The speaker wants to look far into this person, perhaps out of love or curiosity, but gently. The last line is the key: they want to see "without being lit." That suggests wanting to understand someone without exposing or glaring at them, and perhaps without being exposed themselves. The mood is quiet, tender and a little shy. There's longing for closeness, but also respect for what stays private. Speaking and looking blend together, and the poem leaves the wish hanging, unfulfilled, which is part of its soft, hushed feeling.