Did the Mediterranean Sea make it easy for other countries to invade Egypt

It did not, in fact it made it harder. in ancient times, moving an army sizable enough to defeat Egypt would have been extremely difficult. Look at Xerxes' attempted invasion of Greece, it was defeated by an army a quarter of its size. Xerxes' navy lost over 300 ships compared to the 120 lost by the city states of Greece. Just the logistics would be a nightmare, having to feed 70,000 men plus horses.

No, it made it harder.