There are two possibilities here. One is that the seas will overflow and flood the earth. Previously, the mountains were sailed away. Such a huge shift would cause huge earthquakes which will naturally cause the seas to overflow and flood.
Secondly, it can also mean that the seas will become fire. The root sīn jīm rāʾ at its root means to overflow but the Arabs used the same root to refer to fire that lights up the ovens.48 In all of these events thus far, you can see a clear theme of things breaking away from their natural state. The sun is a source of light but its natural state will be taken away. The Arabs used to take care of their pregnant she camels but now they break away from it. Animals by nature flee and break away but here, they remain gathered in one spot. The seas are water, but now they become fire, the exact opposite of their natural state.
When the seas are made to overflow
There are two possibilities here. One is that the seas will overflow and flood the earth. Previously, the mountains were sailed away. Such a huge shift would cause huge earthquakes which will naturally cause the seas to overflow and flood.
Secondly, it can also mean that the seas will become fire. The root sīn jīm rāʾ at its root means to overflow but the Arabs used the same root to refer to fire that lights up the ovens.48 In all of these events thus far, you can see a clear theme of things breaking away from their natural state. The sun is a source of light but its natural state will be taken away. The Arabs used to take care of their pregnant she camels but now they break away from it. Animals by nature flee and break away but here, they remain gathered in one spot. The seas are water, but now they become fire, the exact opposite of their natural state.
48 Al-Muʿjam al-Ishtiqāqī al-Muʾaṣṣal li-Alfāẓ al-Qurʾān al-Karīm, vol. 2, p. 958