I the Lord do keep it; or, guard it (comp. Isaiah 26:3; Isaiah 42:6; Isaiah 49:8; Psalms 121:5). Vineyards were considered to require special watching, since they were liable to damage both from thieves and foxes (So Isaiah 2:15).
It was usual to build towers in them, from which a watch could be kept (Isaiah 5:2; Matthew 21:33). I will water it every moment (compare the threat in Isaiah 5:6, "I will command my clouds that they rain no rain upon it").
The Church needs and receives "the continual dew of God's blessing."