Chemical packs = bad idea. Increased pressure == increased reaction rate == they burn hotter for shorter. People can and have been badly burnt as a result of this. Hot water bottle firstly runs the risk of splitting and causing burns especially if in a drysuit plus exposed to water it will cool down rapidly (1-2 mins) to ambient so be useless.
If you're cold and in a wetsuit, get a drysuit. If you're cold in a drysuit, get a thicker undersuit!
Actually, there are different kinds of chemical packs.
The type of hand warmers that create a chemical reaction with air (don't know what they're called) will get hotter and hotter as the partial pressure of O2 increases (ie, as you go deeper). This is obviously problematic and could even lead to you getting burns.
However, the sodium acetate ones don't react with air or pressure. As you can see from the packaging, it's a closed system. The reaction temperature of these heating pads is a stable 54C and it doesn't change with depth. Obviously that's too hot to put directly on your skin but if you have them inside your drysuit in, for example a breast pocket, where it isn't contacting your skin then they work just fine for keeping your warm during the first 30min of so of a dive.
How do I know this? Because I use them. Have been for years. In the winter the water around here drops to about 2C (sometimes less) and without these heating pads it's very uncomfortable to dive at all in the winter regardless of HOW thick your undersuit is. At least with these things you can make short dives without too much discomfort. I have 4 round ones about the size of my hand. Two of them I put in the breast pockets of an old shirt that I wear over my undersuit and the other two go in the hip pockets. Just before I get in the water I activate them (which can be tricky once you have your drysuit on but it is doable) and it allows me to make dives of about 30-40min even in the dead of winter.
Obviously one of those electric vests is probably a better solution for these kinds of conditions but I can't bring myself to fork out that much money for one. So I just go with the el-cheapo approach and use the sodium acetate hand warmers instead.
R..
P.S. To be honest, I didn't think the hot-water bottle idea was too bad. I may try it this winter to see for myself how well it works.
