Alien Adventures (anthology)

Alien Adventures was a volume in the Doctor Who 2 in 1 books range, collecting The Underwater War and Rain of Terror.

Publisher's summary
One Time Travelling Doctor - Two Alien Adventures

In The Underwater War, the Doctor and his companions are on-board The Cosmic Rover, a spaceship orbiting the water-planet Hydron. Joining the crew, they journey underwater on an apparently scientific exploration. But nothing is as it seems on the high-tech submarine. When a virus infects the crew, the Doctor discovers the ship is hiding a dangerous secret...

And in Rain of Terror, terrible tiny creatures swarm down from the sky, intent on destroying everything on planet Xirrinda. As the colonists try to fight the alien infestation, the Doctor searches for the ancient secret weapon of the native Ulla people. Is it enough to save the day?