The book you are trying to remember is “Beyond the Silver Sky” (original magazine title “Thunder Over Starhaven”) by the British pulp-SF writer Kenneth Bulmer.

Why it matches the details you list

1. Protagonist
   • Vic (sometimes “Vik”) Morgan is a Royal Air-Force fighter pilot who is shot down during the closing months of the Second World War and is entombed in an Alpine glacier.
   • In the late 21st century he is discovered, thawed out and revived.

2. “Up-grading” after revival
   • Because the revivification process has to repair enormous cellular damage, the surgeons take the opportunity to “improve” him – heavier muscle fibre, reinforced bone, and most memorably new liquid in place of ordinary blood, a synthetic amniotic medium able to hold far more oxygen. (Morgan can stay conscious for minutes after his air-supply is cut off – a point that becomes vital later in the underwater sequences.)

3. The war and the commando mission
   • Earth is being bombarded by the people of the planet Klaris whose own land surfaces are turning into desert; they intend to seize Earth as a replacement home.
   • A small strike-ship, carrying a single cobalt-bomb large enough to split the planet, is sent to Klaris; Morgan is put on the team because the surgical alterations make him the one man who can survive almost any emergency.

4. Two intelligent races on the target world
   • Klaris has an indigenous amphibious/oceanic species (“the Sea-Lords”) as well as the land dwellers who are attacking Earth. Klarisian cities are now being built on the seabed, touching off a savage war between land and sea peoples.

5. Capture, the princess and the mercury-mirror telescope
   • Soon after landing the commando party is captured. Morgan is questioned by Princess Shantal, described exactly as you recall – scantily clad, very interested in the handsome alien and using an improvised astronomical telescope whose mirror is a rotating dish of liquid mercury so that she can locate the invaders’ home world.

6. Escape and alliance with the ocean people
   • Morgan breaks out of the under-sea prison, uses his liquid-breathing capability to reach one of the Sea-Lord habitats, gains their confidence and returns with them to wreck the Klarisian under-water stronghold.

7. The atom bomb used to move the planet instead of exploding it
   • In the closing chapters Morgan realises that if the bomb is placed in a particular submarine canyon and triggered as an “engine” rather than a pure explosive, the recoil will nudge Klaris outward into a slightly larger orbit, lowering the insolation and ending the runaway desertification instead of blowing the planet to pieces. (The novel ends just as the device is fired; Bulmer leaves the ultimate success to the reader’s imagination.)

Publication details

• First published as “Thunder Over Starhaven” in Startling Stories, Fall 1954.
• Re-written and expanded for Ace Double D-409 (1959) as “Beyond the Silver Sky”.
• Never re-issued after the 1960s, so second-hand copies often lack the cover – exactly the state in which you found yours.