Re: more time animal spends in water, more highly specialised its forelimbs?

I've got two questions here, that I will try to address in turn.

First, you ask for the reasons that "the more committed to an aquatic 
existence an organism is the more highly specialized its forelimbs are."

In some ways, your perspective is backwards. The best available evidence 
suggests that animals started out in the water, which means that the 
forelimb arrangement you see in most fish (pectoral fins) has a better 
claim to being the "normal" situation. It is land animals that then show 
the specializations in their forelimbs. The forelimbs really became more 
specialized for terrestrial locomotion (walking on dry land) as the animals 
evolved a greater commitment to life away from the water. 

However, I suspect that you were really asking about animals that 
demonstrate a secondary adaptation to aquatic existence. These are species 
that can trace their ancestry to land animals, but which are now more 
suited for life in the water. These are animals such as whales, seals, sea 
lions, otters, polar bears, and brown bears. If you look carefully, you 
will notice that I listed these animals in order from most aquatic (whales) 
to least aquatic (bears). What you are seeing here is a list of animals 
that are adapted to their environments. Each one shows a series of 
specializations (not just in their forelimbs, but in their entire body) 
suited to their lifestyle, and we could continue this continuum of animal 
types to move up into the tress, or out into the plains, or even into the 
desert (as unaquatic as you can get). The point that I am trying to make 
here is that each species is committed to exploiting its part of the 
environment (what ecologists refer to as the "niche"). Evolutionary forces 
shape the animal over time, so that the animal that is better at exploiting 
its environment will produce more offspring that will be equally equipped 
to live in that environment. No particular environmental choice is more 
"specialized" than any other, in the grand sense of things. A horse is as 
specialized for living in the open plains as a whale is specialized for 
living in the open ocean. 

The animals that live on the margins of an environment (taking advantage of 
both) are the animals that might be called generalized. Thus, future 
evolutionary pressures might force the otter into a more aquatic 
environment more easily than you would expect the horse lineage to adapt to 
the aquatic environment. But, as the otter lineage adapted a greater 
commitment to the aquatic environment the "evolved otter" would face new 
ecological competition. To succeed in that competition, the "evolved otter" 
population might choose a strategy of becoming better swimmers - and 
thereby may overtime start to look more like a sea lion. Or, it might take 
the route of becoming larger and more predatory, and end up looking 
something like a polar bear. Both the polar bear and the sea lion are 
specialized aquatic predators. But the sea lion's evolutionary history 
chose a path that made its forelimbs adopt a more fin-like shape, while the 
polar bears followed a path so that their forelimb was able to maintain a 
more "paw"-like shape. Neither animal is more, or less, specialized in it 
environment or in its forelimb, than the other.

The second question is: What features of a forelimb indicate it is suited 
to high speed?" Here I am assuming that you mean high speed on the land, 
and not in the water or in the air. 

Animals that are specifically adapted to running are known as "cursorial." 
The modifications of the forelimb that are generally associated with 
cursorial lifestyles are: reduction in the number of digits (fingers or 
toes), adoption of the digitigrade posture (where the animal stands on the 
tips of the toes/fingers), elongation of the carpal (wrist) and 
antebrachial (forearm) bones, with a typical reduction of the brachium 
(upper arm bone). These modifications increase the length of the leverage 
associated with the limb so that it moves quickly rather than powerfully. 
However, these are just general trends - you really need to look at the 
modifications in the rest of the body. In most cursorial animals, the 
forelimbs are more structured to bearing the body weight, and it is the 
hindlimbs that move the animal forward. So, adaptations for speed would be 
better seen in the hindlimb then in the forelimb.

References:

Alexander, R. McNeill (1968) Animal Mechanics. University of Washington 
Press: Seattle

Hildebrand, Milton (1982) Analysis of Vertebrate Structure. John Wiley & 
Sons: New York.

Radinsky, Leonard B. (1987) The Evolution of Vertebrate Design. University 
of Chicago Press: Chicago