The manual inflation sailing lifejacket is better than nothing - although the aviation approved ones are a bit more durable (and less likely to fail after a hard impact).
Immersion suit is a great idea - a standard recommendation is to wear them when the sea temp is below 15 degrees C.
You can get a personal EPIRB, brilliant bit of kit (and good to always have when flying). Have a look at them on google (make sure you get one that is waterproof). Marine suppliers are good for these (I assume that is where you got your lifejacket).
If you have one, take your marine band VHF handheld (a call on Channel 16 is always worth it).
If you are going to do this regularly, then do a dunker course - it really does make a difference should the worst happen.
FFF