Albert Barnes Commentary Psalms 89:9

"Thou rulest the pride of the sea: When the waves thereof arise, thou stillest them." — Psalms 89:9 (ASV)

You rule the raging of the sea - The pride; the anger; the lifting up of the sea. That is, when the sea is raging and boisterous; when it seems as if everything would be swept away before it, you have absolute control over it. There is, perhaps, no more impressive exhibition of divine power than the control which God has over the raging waves of the ocean: and yet this was the power which Jesus exercised over the raging sea of Galilee - showing that he had the power of God (Mark 4:39–41).

When its waves arise - In the lifting up of the waves; when they seem to raise themselves up in defiance.

You still them - At your pleasure. They rise no higher than you permit; at your command they settle down into a calm. So in the troubles of life - the storms - the waves of affliction; they rise as high as God permits, and no higher; when he commands they subside, and leave the mind as calm as the smooth sea when not a breath of wind moves over its surface, or makes a ripple on its placid bosom.