score:4
You would rather need a 220 to a 110 converter. I have been using this converter(on a daily basis for over a month now) in Australia (India and Australia have the same voltage/power levels I am told) and all my US appliances work well. It has a little bit of a fan noise in a very quiet room, but that doesn't bother me.
Also, you need this only if your toothbrush does not have a symbol which says it will work between X to Y (usually 110~220) voltages
Upvote:6
You may be better off with a 220V certified rated charger, rather than converter. They should be freely interchangeable with 110V ones, as long as toothbrush is concerned.
For Sonicare:
http://www.amazon.co.uk/Philips-Sonicare-FlexCare-Toothbrush-Genuine/dp/B00DGTGTXY
For Waterpik:
http://waterpikukonline.co.uk/replacement-parts/wp-450-chargers
The problem with Waterpik appears to be that there's no easy way to locate the p/n of Indian/euro power brick variety of their power supply (the link above leads to British variety, which, albeit being 220v, will not fit into Indian wall socket).
Why this is important:
Any sort of wall socket adapter is inherently unsafe, especially when used in bathroom environment. Even more so, voltage converting adapters are heavy and thus prone to partially falling out from a socket, exposing live leads.
Having said that, many hotels around the world do have an "universal" shaver outlet in the bathroom with 110v setting selectable by a switch (so American equipment can be used as is).