The first thing you need to do is get it out of the water if you have the rootball submerged. That's a good way to wet a containerized plant if it has become too dry, but a rhododendron can drown in as few as 6 hours under water.

Always set the plant so that its root-ball is no lower than it was before it was dug for transplanting. It is preferable to set it even a few inches higher so that it will not settle to an unhealthy depth...these have a network of surface feeder roots that cannot function if planted too deeply.

The soil should be acidic (low ph not alkaline), and fairly high in organic content not only to aid in supplying nutrients, but to help keep the medium loose and friable, aid in the aeration process. Shredded bark, composted leaves and other composted organic materials are soil conditioners.

At no time should water be allowed to collect or stand in the planting site but at the same time, rhododendrons need to have the soil around them moist during dry weather. A good way to do this is to use a light mulch of the root zone after planting.