When things in the land were bad, Isaac did not trust in Yahweh. Instead, he ran to the safety of Egypt. Even when Abraham's descendants did not trust in Yahweh's faithfulness to fulfill his covenant, Yahweh remained faithful to his covenant and blessed Isaac. (See: rc://en/tw/dict/bible/kt/trust, rc://en/tw/dict/bible/kt/faithful, rc://en/tw/dict/bible/kt/fulfill and rc://en/tw/dict/bible/kt/covenant)
In the ancient Near East, wells were very important strategically. Therefore, they were a sign of power and a sign of God's blessing being upon Isaac.
Isaac was afraid the Egyptians would see him as a foreigner and upon seeing his beautiful wife, they would try to kill him. This would free Rebekah to marry someone else. Apparently, it would have been easy for them to kill a foreigner without punishment. If she was Isaac's sister, they would have shown favor to him. This is the same thing his father, Abraham, did. (See: rc://en/ta/man/translate/figs-explicit and rc://en/tw/dict/bible/kt/favor)