Bede

The water was fifteen cubits higher than the mountains which it covered. Indeed, seven and eight make fifteen; seven pertains to the rest of souls which is to be after death, since the Lord certainly rested in the tomb on the seventh Sabbath; but since He rose from the dead after the Sabbath, that is, on the eighth day, the number eight very rightly indicates the time of our resurrection. …

Commentary on Genesis (Hexaemeron) ↗