I have a Tosh e740 as well, yes it is possible, although it has limitations (screen size being the main one), the screen really is too small to be of any practical nav use, and I've never actually used it on the boat.
I use the Maptech software which is run on the PC, and this sends the map to the memory map software (again by Maptech) on the PDA.
For GPS input I have the Pretec CF card GPS.
Given the limitations it works ok, although for all said it is an expensive way to go, (Maptech £99, Memory Map £69, Pretec GPS £150, especially if you only want to use it in a marine environment, if you have other uses ie land based navigation, route finding, walking etc it mitigates this a bit.
For marine use I would rather use a dedicated chart plotter, with the normal paper charting running alongside, PDA's are a little fragile in a marine environment I feel.
The Tide software that is avail is very good though, I use the Belfield software (tideplotter 2003) good value at £10.00 ish, accurate and works really well on PDA.
Also use it for weather data from internet via mobile phone, or WiFi, and email.
Expansys.com is a valuable source of bits and info, or l8shop.net.