Solution: (1) Let the radius OC⊥AB, the vertical foot be point D, and connect OA, then CD is the arch height ... (1).

∴ oc ⊥ abad =1/2ab ......................................... (2 points).

Ao = 0.5, AB= 0.6, ∴ OD = 0.4 ......................................................... (1min).

∴ CD = 0. 1, that is, the water depth at this time is meters ................................. (1).

(2) When the water level rises to m width, the straight line OC and MN intersect at point P. 。

That's the truth, ................................................... (1 min)

(i) When MN and AB are on the same side of the center of the circle, the rising height of the water surface is 0.1m; ................. (2 points)

(ii) When MN and AB are located on opposite sides of the center of the circle, the rising height of the water surface is 0.7m .............................................. (2 points).