Whilst Nicads do not normally vent whilst being charged, they will if overcharged. Nearly all modern Nicads are "vented cells" and if you are using non-vented ones then you should stop using them because they are not suitable for use in models, the only possible exception being Rx Nicad packs.
If you are using Nicads to power your model e.g. 6/7 cell sub C pack, then it's "good practice" to take the pack out of the boat to charge it. If the worst should happen and the pack becomes over charged to the extent it blows up it will destroy your boat. On the real doom and gloom side, the gasses and solid particles given off from an overcharged Nicad are VERY poisonus and can kill!! Cadmium is deadly......be warned!!