The earth we live on, with gravity
510.1 million km2 total surface area, of which Land area is
148.9 million km2. So we are already at the 71% mark.
Add some global warming, and we might reach 75-80% on this earth itself.
So, coming to your planet with gravity
0.7g. Water has a mass of its own. With lower gravity, we are going to see less cancellation of the viscous force that water exerts on itself. Also, there will be a general reduction in pressure that a column of sea water exerts on the water surrounding it.
Consequently, the sea water will be much more "freer", and the waves will have more energy than they have today. More energetic waves mean more more erosion of the rocks, so you will come to see the seas engulfing all but the hardest of rock surfaces.
Also, lower gravity means the rocks themselves may not be as hard (as they wouldn't have set as much under their own weight). So this is cause for further erosion of rocks, as they are softer.
So yes, you can expect to see 90% of planet's surface submerged.