Ask Mr. Smarty Plants

Which is more important? The pond or the oaks? We believe you are creating a large amount of impervious surface above those oak roots, just as if you were paving over the yard. As you say, the oaks were there first, and only you can decide if it is worth the risk.

An oak tree root system is extensive but shallow. The ground area at the outside edge of the canopy, referred to as the dripline, is especially important. The tree obtains most of its surface water here, and conducts an important exchange of air and other gases. Any change in the level of soil around an oak tree can have a negative impact. The most critical area lies within 6 to 10 feet of the trunk. No soil should be added or scraped away from that area. Construction activity is a great threat to trees. Do not allow any piling of materials, waste, etc. in the dripline area.

Paving should be kept out of the dripline and no closer than 15 feet from the tree trunk. If at all possible, use a porous paving material such as brick with sand joints, open bricks, bark, gravel, etc., which will allow some water penetration and gas exchange. Even with porous paving, the area around the trunk-at least a 10 foot radius-should be natural and uncovered

Water Gardening from How To Articles. From that article:

"Finding an ideal location for your pond should be your first consideration. It is possible to have a water garden in considerable (although not dense) shade, but leaves falling from nearby trees are a maintenance headache. Your largest selection of aquatic plants are those that grow in plenty of sun."

Previous Mr. Smarty Plants answer - Shade Trees Not Invasive to Foundations and Driveways

Previous Mr. Smarty Plants answer - Distance of oak tree to existing driveway in San Antonio

So, do we think your plan is okay? Sometimes when you can't find an answer to a question, it's because the answer is "no." There is absolutely no reason why you cannot do precisely as you have outlined. The trees might survive, but if they don't, you will need a Time Machine to fix it. You have asked our opinion. Since we live in dry, arid Texas, we would vote for the tree every time.