You should probably get a louder smoke decetor if you can barely hear it upstairs.
I'm going to go with the DIY approach;
For the water sensor, I'd look into the possibility of linking the basement alarm to a speaker upstairs. I've no idea what kind of alarm you're looking at or what the electronics are like. Theoreticaly, you can jump off the audio signal just before it reaches the speaker. Send the audio signal through an amp (located close* to the alarm, preferably where it won't get wet) and connect it to a speaker upstairs.
I would never try to mess with a smoke detector I rely on, but a water sensor...buy two and have fun.
*the amp is to overcome voltage drop in the new cable, I doubt that the sensor electronics will be capable of driving a seperate speaker with at least 30 ft of cable between it.