Parashas Shemos
וְלֹא־יָכְלָ֣ה עוֹד֮ הַצְּפִינוֹ֒ וַתִּֽקַּֽח־לוֹ֙ תֵּ֣בַת גֹּ֔מֶא וַתַּחְמְרָ֥הֿ בַחֵמָ֖ר וּבַזָּ֑פֶת וַתָּ֤שֶׂם בָּהּ֙ אֶת־הַיֶּ֔לֶד וַתָּ֥שֶׂם בַּסּ֖וּף עַל־שְׂפַ֥ת הַיְאֹֽר׃
When she could hide him no longer, she got a wicker basket for him and caulked it with bitumen and pitch. She put the child into it and placed it among the reeds by the bank of the Nile.
Torah Temimah points to Sotah 12a (verified).
תבת גמא — \ldots
English: An ark of reeds — chosen because reeds float and are soft.
Custom
The ark was made of material suited to floating safely on the Nile.