Excessive rainfall will cause fruit split .
This has been a big problem for us in Florida thanks to the 24 inches from Fay .
Too much water can make citrus fruit taste bland .
Growers recommend removing fruit from young trees for the first two years after planting .
This encourages a strong root system .
Wait until the next crop for better fruit .
Nothing you can do to make citrus sweeter .

Well, many citrus get sweeter the longer they are on the tree (until they start rotting, or course). Try one of the other two remaining oranges in January and see if it tastes any different.

I have heard that also . . .

From the UF link posted below :

Navel orange

Harvest October to January

Navels differ from other oranges by having a rudimentary secondary fruit embedded at the blossom end of the fruit. Premature yellowing and rot of this secondary fruit often results in premature fruit drop. Fruit peels relatively easily, sections well. If juiced, drink within several hours before a bitter flavor develops. Tends to require more precise irrigation and nutrition management. Two periods of fruit drop, early- and late-summer, account for 15-20 percent of the crop in some years. Cara Cara navel is used for salads because of its near-crimson flesh. Seeds: 0-6 per fruit.

Here is a link that might be useful: Florida Dooryard Citrus Guide

Too much water will make most fruit less sweet. In some cases the difference between excess water and mild water stress can be amazing. The stonefruit in my greenhouse is very sensitive to water. Too much water and the fruit is large, beautiful, and bland to awful. Give it a little water stress and the quality can be amazing. My sweetest Washington navel were last year when the tree was water stressed. This yr I haven't tasted the fruit yet, two months until it is ripe, but it has had more water and is much larger. I've leaned that extra large fruit often goes along with bland taste.

You already know the fruit was over watered. Try to impose some mild water deficit next yr.

The Fruitnut

dittos all the above. Also bear in mind that oranges improve over the years. A ten year old tree will have much better fruit. A twenty year old tree will be even a little better yet. Old mature trees definitely produce the best, sweetest fruit.

The fruit on my satsuma wasn't particularly sweet. I asked a learned person. She said phosphorus makes the fruit sweeter. I haven't tested this, but I will try this next year. If the fruit doesn't improve, I'll replace the tree with a better variety.

A local edible nursrey in my area told us to use 1 cup of epsom salt every six weeks to improve the sweetness of the orange. This will be my second year of harvesting oranges off my tree (25' Navel, or so I've been told ... it came with the house and last years crop was weak because the tree was neglected from April-October of 2007 ... foreclosed house that I purchased so nature took care of it for 6+ months. Oh, just a little FYI, this is my first citrus tree so I'm just passing on what I've been told. I won't know if it works for a few months ... I don't even know when to begin picking the oranges off a tree so I'll be doing a lot of reading on this site and hopefully grow a green thumb in the process.

I do not know of anything that will sweeten fruit .

A young tree might produce less than desirable fruit and improve with age .

An older tree starting to decline might produce fruit that is not as sweet as it once was .

Disease can affect citrus fruit and cause issues with flavor .

But I have seen no evidence that shows that you can apply a product and magically get sweeter , better tasting oranges .

Central - sounds like your tree is just too young . Give it time and TLC .

Most of the time, a tree will produce a poor flavoured crop for the first year or two, but it will continue to get better (like fine wine haha) It will produce nicer tasting ones as it grows up. It was just starting itself off this year :)

I've got surprisingly sweet fruit this year, it can be due to a few reasons. Thanks to this site I've learnt to make my own liquid kelp, the orchards get 5 liters of kelp every 25- 30 days. S has a big influence on taste as well as P en for the first time I've split up all elements over a 7month period also concentrating on the K;Ca;Mg balance en changing for Urea N to Nitrate N split up to a portion every month as my mind told me the plant would need it. I've added more water, for shorter irrigations more often due to silty soil with a lower ability to hold water. I think its a combination of things not excluding humic acid, instead I added some organic material under the trees.(Palmer Navels)

I know this won't help anyone who has trees already, but I think I read that certain rootstocks might contribute to sweeter fruit. I wonder if that holds for grafted (fruit salad type) trees as well?