- The spectrum will be filtered
This causes a drop in efficiency. For single junction cells, which are very tolerant to changes in spectrum, the drop will be much less than for multi junction solar cells which are designed for particular spectral conditions.
- Leakage current
The IV characteristic will change because the surface resistance will decrease causing a leak current. This will also reduce efficient.
- Reflection at the surface will change.
Reflection occurs at the interface between materials of different refractive index. Semiconductor have a high index, say around 3.5, so the reflection is high. For these reasons solar cells have surface antireflection treatment which tries to place a materials of intermediate index between the surface and air (#1). If you place this in water you will skew this behaviour away from the design specification.
(#1) the details here are quite fascinating, I slightly over simplified this point.