For an aircraft unable to reach a runway on land, ditching in water is a common and, generally, highly-survivable option; however, for the space shuttle, according to Wikipedia, "...an ocean ditching was not survivable" (although it lists no source for this claim). What was it about the space shuttle that made it incapable of ditching into water in a survivable manner?
The shuttle lands at 220 mph and would break up. This is 40% faster than airliners with twice the energy to dissipate and the shuttle structure is a great deal lighter. The orbiters were designed to go to space, ending each mission of their 100 mission lives by landing on a smooth runway. Airliners are designed for forty to a hundred thousand landings or more and they don't always fare that well in water either.
The wiki line provides no citation nor any elaboration but is largely correct. It was found that the orbiter actually had a favorable shape to ditch,
The Langley report does state that the Orbiter shape and mass  properties are good for ditching, but given the structural problems and deceleration loads, that is little consolation.
This was touched upon in the Challenger report, ultimately the high loads would simply have not lead to a survivable impact which had to do with both the high approach speeds and potentially insecure cargo mass (from a deceleration standpoint).
In 1974 and 1975, ditching studies were conducted at Langley Research Center. Although test limitations precluded definitive conclusions, the studies suggested that the loads at water impact would be high. The deceleration would most probably cause structural failure of the crew cabin support ties to the fuselage, which would impede crew egress and possibly flood the cabin. Furthermore, payloads in the cargo bay are not designed to withstand decelerations as high as those expected, 2 and would very possibly break free and travel forward to the crew cabin. The Langley report does state that the Orbiter shape and mass  properties are good for ditching, but given the structural problems and deceleration loads, that is little consolation.
Orbiter ditching was discussed by the Crew Safety Panel and at Orbiter flight techniques meetings before the first Shuttle flight. The consensus of these groups was that (1) ditching is more hazardous than suggested by the early Langley tests, and (2) ditching is probably not survivable.
This view was reiterated in the September 9, 1982, letter from Griffin to Abrahamson:
"We also suggest no further effort be expended to study bailout or ditching. There is considerable doubt that either case is technically feasible with the present Orbiter design. Even if a technical solution can be found, the impact of providing either capability is so severe in terms of cost and schedule as to make them impractical."
There is no evidence that a Shuttle crew would survive a water impact. Since all contingency aborts and all first stage abort capabilities that are being studied culminate in a water impact, an additional provision for crew escape before impact should also be considered.
Astronaut Paul Weitz expressed this before the Commission on April 3, 1986:
*"My feeling is so strong that the Orbiter will not survive a ditching, and that includes land, water or any unprepared surface....
"I think if we put the crew in a position where they're going to be asked to do a contingency abort, then they need some means to get out of the vehicle before it contacts earth, the surface of the earth."*
NASA did publish a study about ditching analysis of airborne vehicles here (focusing on the shuttle) its science heavy but an interesting read on the subject.
Lowering ground speed increases survivability by the square of velocity. In other words, an off runway landing at 100 knots has only 25% of the energy to dissipate compared with a 200 knot impact.
Deltas have the ability to maintain lift at a very high angle of attack, as demonstrated by the XF-92A holding off the runway at 67 mph, 100 mph slower than previous landings.
The inability of the Shuttle to ditch may have had something to do with its ability to re-entry the Earth's atmosphere. The first Shuttle flight revealed a strong pitch up tendency due to the ionization of air under the nose from the extreme heat. A forward CG adjustment helped control this.
Also the normal landing technique, for the runway, was to come in at very high speed, with a shortened nose gear, allowing the nose to drop to negative angle of attack and "stick" it to the runway after touch down. (No bounce and go around here).
This craft simply was not designed for slow, high AOA landings on water or anywhere else. Even if it could pitch up, it's great weight would put tremendous stress on any structures while decelerating during a "ditch".
An ejectable crew capsule may have given them better odds.