Are the Etruscan words "tisś" (lake) and "tusna" (swan) related? I can imagine the word for swan coming from the word for lake, as swans live at lakes.
The simple answer is we don't know. A lot of Etruscan etymology is opaque because of our limited knowledge of the language.
The word tusna is attested mostly as a name; there's one mirror where a giant swan is labelled tusna, which Puhvel 1984 argues is probably a word for "swan" because swans in mythology usually don't have names. He derives it from Italic *lousnā "white, shining" (the ancestor of lūna), with the famous Etruscan alternation between
/l/. But whether this word even means "swan" is not entirely clear—even if the word is borrowed from *lousnā it could be the name of this particular mythological being instead of the general word for swans.
The word tisś is somewhat better attested, often in compounds like naitis. Fournet suggests a cognate to Hurrian šiwe "water" but explicitly marks this as "unclear connection".
In the end, a lot of etymological work on Etruscan is restricted to hypotheses and various levels of educated guessing, because the corpus we have is so sorely limited. When we don't even know for sure what the words mean, it's difficult to figure out connections between them.