Simply the glass degrades because it dissolves in the water.
First the how. There are two factors which impact this process which is the dissolution of sodium and the dissolution of silica. The dissolution of sodium occurs first as evidenced by a plot of pH vs time for a glass powder in water and proceed via electrophilic attack:
$$\ce{Si-O-Na_{(glass)} + H2O_{(l)} \longrightarrow Si-O-H_{(glass)} + OH-_{(aq)} + Na+_{(aq)} }$$
The second step is the dissolution of the silica which proceeds via nucleophilic attack:
$$\ce{(-O-)2Si(-O-H)2_{(glass)} + H2O_{(l)} <-> Si(OH)4_{(aq)} }$$
The solubility of the silica is, of course, low, but measurable and significant. This process at high temperature increases the solubility of the silica (hydrothermal) and is how natural quartz crystals were formed (hydrothermal vents).
Second you say that degradation occurs slower in seawater than distilled water and that, in fact, is correct. At first, one might suspect that the dissolved sodium reduces the dissolution due to a common ion effect or reduced chemical potential but is actually not the case. The operative word here is seawater. Seawater already has a large amount of dissolved silica in it from the rocks and sand that lines coastlines and ocean beds such that it is saturated. Since the second reaction is reversible the increased concentration prevents the silica from being leached appreciably faster than it can be redeposited thus reducing the corrosion of the fiber
