Making a few additions to Tim C's excellent answer:
Incorporeal creatures have an innate sense of direction and can move at full speed even when they cannot see.
This line, which burrowing creatures lack, implies that creatures in objects are Blinded while fully submerged (i.e., not attacking). Since burrowing creatures lack this line, in addition to the usual penalties to AC and Dexterity for being Blinded, they "must make a DC 10 Acrobatics skill check to move faster than half speed". It would be reasonable to waive this requirement for creatures with Tremorsense (I certainly would!), but doing so is not RAW.
Tim C's interpretation that other creatures will also have Cover from a burrowed creature is not mentioned in the Incorporeal rules, but stems from the Cover rules:
To determine whether your target has cover from your ranged attack, choose a corner of your square. If any line from this corner to any corner of the target’s square passes through a square or border that blocks line of effect or provides cover, or through a square occupied by a creature, the target has cover (+4 to AC).
When making a melee attack against an adjacent target, your target has cover if any line from any corner of your square to the target’s square goes through a wall (including a low wall). When making a melee attack against a target that isn’t adjacent to you (such as with a reach weapon), use the rules for determining cover from ranged attacks.
However, note that according to these rules other creatures do not have Cover from a burrowing creature's ranged attacks, only their directly adjacent melee attacks.
Finally, the implications of having Total Cover should be made clear.
You can’t make an attack against a target that has total cover.
Similarly, burrowed creatures do not have line of effect to other creatures while fully submerged, as "line of effect is blocked by solid barriers", meaning that they are unaffected by most spells (unless said spells are cast as a readied action). As a result, any creatures wishing to attack a fully submerged burrowed creature (perhaps to take advantage of their Blinded status) must first destroy or bypass the wall that they are submerged in before targeting them, or ready an action to attack when the burrowing creature emerges.