Don't think the spars were designed to be 100% watertight.
You need to take an endcap off to get the water out; remove the pins that are holding the cap in place. You will probably need new pins (buy a bag from a Vanguard dealer) to get back to normal. I believe that people also use rivets, but I am not sure about that.
Sunfish and Sailfish often get water in the spars, usually in the gaff (the upper spar) the boom (the lower spar) and/or the mast. As Wavedancer indicated, remove one if the plastic end caps and see if the spar will drain. Some spars had a cork fitted to the inside of the spar and you may have to remove that cork as well and then replace it (or not) when you have all the water out; the cork is not essential to the proper functioning of the spar. Water gets into the spars through
rivet holes, rust holes or leaky end caps and it can add quite a bit of weight to what is suppposed to be a light, alum. spar.
I believe (?) that the cork inserts at the end of the spars were to make them more watertight, but when the spars do leak, the cork seals trap the water inside. Hence, if you remove one of the plastic end-caps and then have to remove the cork seal, don't replace the cork when you re-attach the plastic end cap. I do not believe the cork is required by class rules and I do not believe the newer boat have the cork seals.
As for outhaul tension, if you plan to race your Sunfish, you will want an easily adjustable outhaul to adjust the tension on the foot of the sail (i.e, along the boom) and a similar rig (called a Cunningham) to adjust the tension on the luff of the sail (i.e. along the gaff.) These adjustments are just lines that lead to clam cleats in the boom. For recreational sailing, I would simply tension the luff of the sail so there are no wrinkles on the luff and then dead end about 2' of 3/16" diameter line to the clew
of the sail (i.e. the outboard end of the foot) and then run this line through the plastic end cap fitting and back to the clew grommet. Pick the proper tension and secure with a few half-hitches around the lines between the end cap and the grommet. For heavy wind, tighten the outhaul (to flatten the sail.) For light air, ease the outhaul to put more draft in the sail. Experiment a bit and tune accordingly.
As far as I know, the cork is not required by class rules. If it is, I've been cheating for a long time. Ooppps, soory.
As far as water goes, I've drilled a small hole at the bottom of each spar and then put a piece of sail-repair tape over it. That way, it's mostly sealed up.... but if water does get into it, I can simply drain it out. In my experience though, water in the spars has never been a problem.