Can CANDU6 reactors be refitted with cooling towers? Only Enhanced CANDU 6 seem to offer this.

Romania's two units are not CANDU 6.

"In March 2021 Nuclearelectrica stated it expected to commission unit 3 by 2031, starting construction in about 2024. The largely-new reactors will be updated versions of the CANDU 6, but not the full EC6 version, since the concrete structures are already built. Unit 3 was reported to be 52% completed and 30% for unit 4, though in 2017 the reported figures were 15% and 14%. They would have an operating lifetime of 30 years with the possibility of a 25-year extension."¹

CANDU 6 is open cycle. EC6 can work in a closed cycle with cooling towers. One just doesn't simply make alterations to nuclear power plant designs without the appropriate design, testing and certification. Unit 1 was also completed in 1996, so it's at near the end of its planned operating lifetime.

The Indian PHWR developed from the CANDU can also work in a closed loop.

For Romania there are other solutions, like redirecting flow through the Old Danube branch, which they already attempted through rock blasting and sinking a bunch of rock filled barges in order to buy two more weeks before shutting down the plant. This, of course, comes at a cost for shipping traffic.

1. https://en.wikipedia.org/wiki/Nuclear_power_in_Romania