Do I need a water trap?

There is, in my opinion, a fundamental flaw in the design of the bagpipe.  The issue is that the blowstick effectively "aims" the humidified, spit-laden air right at the chanter stock with the result that first obstruction that the moist air hits is the chanter reed. Thus, the reed gets wetter as it is played longer. This (IMHO) is the major source of lack of stability for many (otherwise) good pipers. Ideally the air reaching the chanter reed would be "at a controlled humidity" before it reaches the chanter.

The other major issue is that spit contains enzyme.  These enzymes are known to eventually digest the bag just below the blowstick.   This is a common reason for failure of Canmore and/or Ross bags.

For these reasons, regardless of the type of bag you play, a water trap may be an advantage. I've divided them into three types:

A Spit Diverter: A simple "elbow" fitting at the bottom of the blowstick stock can divert spit-laden air away from the reed and towards the back of the bag where it will "dry out".

The Cup Trap: These are sometimes described as arrangements of a cork and short piece of tubing that forms a cup in the bottom of the blowstick stock. At least one manufacturer provides a press-fit version for their pipes. The cup-type water trap design only captures the biggest droplets of spit that come out of the blowstick. I haven't found these to be particularly useful.

The Tube Trap: A long tube provides a tortuous path and diverts the humidified air to the back of the bag. The air then travels forward an has a lot of contact with the walls of the bag at relatively low velocity This arrangement allows for the trapping of spit and allows for maximum dehumidification of the air before it reaches the chanter or drone reeds. It effectively addresses both the problems described above and does a nice job of preventing the chanter reed from getting wet. The trap is emptied after playing or as necessary to remove condensed moisture.

There are several marginally adequate add-in designs available commercially. Some fit inside your chanter stock. Some include a replacement stock. Most of them tend to kink at the bottom of the blowstick. Homebuilt versions are readily concocted. One possibility involves cutting 1/2" NPT pipe threads into the bottom of the chanter stock and using a 90 degree elbow to avoid the tendency to kink. Set up with large (e.g 3/4" ID) plastic tubing, these really work.

I've tried connecting a long tube type of trap to the chanter stock and can obtain "essentially" the same effect as a water trap because it takes only "dried" air from the back of the bag to the chanter. The dampness stays in the bag and you functionally loose the ability to empty the trap. This arrangement tends to kink when storing the pipes, but is otherwise worth considering.

As someone speaking with the voice of experience, don't bother trying to use corrugated tubing to prevent kinking. This has the effect of creating a whistling mess known as a corrugaphone.