Air carrier Captain reported sliding when parking the aircraft due to water and slush being on the ground surface at PWM airport.

Narrative

Uneventful until arriving to Company ramp at PWM. Approaching Gate X; silhouette was clear with Marshaler wands up. We observed what appeared to be slush in the silhouette; as a result I taxied approximately 5 knots; slow; steady cautious. The gate has a down slope toward the parking position. With approximately 30 feet to go; I began braking to further slow. As I applied the brakes; there appeared to be no change in acceleration; I continued to steadily increase brake pressure as it appeared the aircraft appeared to accelerate. With full brake pressure applied; within approximately 10 feet of the planned stopping position; and no indication the aircraft was stopping; I extended the Thrust Reverses; (TR's) Passing the expected stop position; I began applying reverse thrust; however; it was at this point; the aircraft began to decelerate to a stop. It turns out the Reverse thrust never went above reverse idle; before stopping. After stopping; the tug was connected to reposition the aircraft. Debriefing the Maintenance Personnel who was also the Marshaler; he observed the main gear began sliding side-ways during the approach to stop. No damaged was noted by aircrew or maintenance; nor were any flight manual limits exceeded; but all parties acknowledged it was a very close call. The Maintenance Personnel said he requested salt and sand support from the airfield prior to our arrival; but was denied. It seemed PWM was aware of the ice fouling the ramp; which was under the water and slush observed on the surface. Failure to notify aircrew of the hazard is a cause. However; had the airport authority properly treated the ramp and taxiway area; the hazard would have been abated entirely.If ice is known to foul the parking spot; send a note during the in-range ACARS message to expect a tow in. If that window of opportunity is missed; drive the tug out and give marshaling directions to set brakes and shutdown. Of course; having the parking area properly treated would have solved most of the problems.