Brew Fail Diagnosis Help

I have no experience with Munton's extracts. However, after a little research, I see two potential problems.

Not enough fermentables

Since you didn't have a hydrometer to measure the OG with I have to speculate. The can of extract weighs 1.8 kg which, if it is a typical liquid malt extract, should yield 5 gallons of 1.029 wort (reference). You mentioned that the container said the OG should be 1.040 to 1.044. Did you add extra sugar?

Neglecting the extra fermentable sugars would account for the weak & watery taste. What about the hop bitterness? Is it out of balance with the maltiness? Since the can contains all the hop flavor, but not enough malt, it should be on the bitter side. Wheat beers are generally lightly hopped, so this may not be apparent.

The lesson here is carefully measure volumes and specific gravity.

Infection

Assuming you followed the directions, an infection could have consumed sugars that brewer's yeast normally would not. This may result in an alcoholic, watery beer. Are there any hints of infection, such as sourness or vinegary flavors?

Try, try again

Canned beer kits are a great way to start out with the hobby. As you learn more and gain experience your beers will improve. There are good resources here when you are ready to upgrade to a first-time setup or go straight for all-grain. Keep it up.

Munton's beer kits always make light beers. They're not bad, but you have to add some more extracts on your own in order to have a good result. You can see that clearly by comparing the munton kits with the brewferm kits: while they use about the same amount of malt extract, munton's suggests you use it for about double the wort quantity brewferm does. Personallly, when using Munton's kits, I choose either to add some extra DME and probably hops, or use more than one kits.

There sounds like a lot of variables here, any one of which could have resulted in characteristics you describe. Here are a few tips for your next batch.

1. Don't give up, you will be surprised how much better your next batch is.
2. Pay close attention to the liquid volumes. Pre-mark the inside of the bucket with a sharpie.
3. If you followed the instructions and got the right volumes, the gravity reading would be spot on.
4. If its still bubbling vigorously, then its probably still fermenting, don't rush to secondary.

Point 4 is especially true with a wheat beer that isn't expected to clear up anyway. There is a lot of discussion about yeast off-flavors when left in the primary too long but most people agree that it has to be way long to have a significant effect. Also less of an issue in a wheat beer.

And now the most important tips:

1. If you have a local brewstore, even though they may be more expensive than the online places, go there and make friends with the owner and other customers. They are you best source of advice. Next best is a local brew club.
2. Grab a book like Dave Miller's Homebrewing Guide or Charlie Papazian's Joy of Homebrewing. These are invaluable resources and no brewhouse should be without them.

Could be a lot of factors at play here, but lacking a hygrometer to take an initial gravity reading is likely a big factor here. What was your final gravity? Adding water is a non-issue from this condensed-wort brewer. I just recently picked up a 7.5 gallon brew kettle, and for the past 5 or so years have been using a 2-3 gallon boil with top-off to 5 gallons using clean, filtered water. As Arlo427 said, we need more info. What was your ingredient list and process?