Imperial Japanese Navy (IJN)

Striking Force: 1st Carrier Airfleet
    Vice-Admiral Chuichi Nagumo in Akagi

Cardiv 1
Akagi (F)—Capt. Kiichi Hasegawa
air group: 27 A6M2, 18 D3A1, 27 B5N2

Cardiv 2
    Rear-Adm. Tamon Yamaguchi
Hiryu (F)—Capt. Kaku
air group: 21 A6M, 21 D3A, 21 B5N
Soryu—Capt. Ryusaku Yanagimoto
air group: 21 A6M, 21 D3A, 21 B5N

Cardiv 5
    Rear-Adm. Hara
Zuikaku—Capt. Ichibei Yokokawa
air group: 18 A6M, 27 D3A, 27 B5N
Shokaku—Capt. Takatsugu Jojima
air group: 18 A6M, 27 D3A, 27 B5N

3rd BB Squadron
    Rear-Adm. Gunichi Mikawa
Kongo (F)
Haruna
Hiei
Kirishima

8th Cruiser Squadron
    Rear-Adm. Hiroki Abe
Tone (F)
Chikuma

Desron 1, 1st Fleet
    Rear-Adm. Sentaro Omori
Abukuma

Desdiv 17
Urakaze—Cmdr. Nagayoshi Shiraishi
Tanikaze—Cmdr. Motoi Katsumi
Isokaze—Cmdr. Shunichi Toshima
Hamakaze—Cmdr. Tsuneo Orita

Desdiv 18, Desron 2, 2nd Fleet
Kasumi—Cmdr. Kiyoshi Tomura
Arare—Cmdr. Tomoe Ogata
Kagero—Cmdr. Terumichi Arimoto
Shiranuhi—Cmdr. Jisuo Akasawa
Akigumo—Cmdr. Shoohei Sooma (attached directly from Cardiv 5, 1st Airfleet)

Malay Force (Raids into Bay of Bengal)
    Vice-Adm. Jisaburo Ozawa

Chokai (F)

7th Cruiser Squadron
    Rear-Adm. Takeo Kurita
Kumano (F)
Mikuma
Mogami
Suzuya

Cardiv 4
    Rear-Adm. Kakujo Kakuta
Ryujo
air group: 16 B5N

Desron 3, 1st Fleet
Yura

Desdiv 11
Fubuki—Cmdr. Masao Yamashita
Shirayuki—Lt. Cmdr. Rokorou Sugawara
Hatsuyki—Lt. Cmdr. Junnari Kamiura
Murakumo—Cmdr. Hideo Azuma

Desdiv 20
    (replaced Desdiv 11 on April 3rd-4th, 1942)
Amagiri—Cmdr. Buichi Ashida
Asagiri—Cmdr. Nisaburo Maekawa
Shirakumo—Cmdr. Toyoji Hitomi
Yugiri—Cmdr. Masayushi Motokura

Submarine Force
    (Positioned west of India)
I-2
I-3
I-4
I-6
I-7

Royal Navy (RN)

Eastern Fleet
    Vice-Adm. Sir James Sommerville
Force A (Fast Force)

Aircraft Carriers
    Rear-Adm. D. W. Boyd
Indomitable
 air group: 9 Sea Hurricanes, 12 Fulmars, 24 Albacores
Formidable
 air group: 16 Martlets, 21 Albacores, 1 Swordfish

Battleship
Warspite (F)

Heavy Cruisers:
Cornwall*—Capt. Manwaring
Dorsetshire*—Capt. Agar

Light Cruisers:
Emerald
Enterprise

Destroyers:
Napier (RAN)
Nestor (RAN)
Paladin
Panther
Hotspur
Foxhound
 

Force B (Slow Force)
    Vice Adm. Willis

Battleships:
Resolution (F)
Ramilles
Royal Sovereign
Revenge

Aircraft Carrier:
Hermes*—Capt. Onslow
air group: 12 Swordfish

Light Cruisers:
Caledon
Dragon
Jacob Van Heemskerck (Dutch)

Destroyers:
Griffin
Norman (RAN)
Arrow
Vampire* (RAN)
Decoy
Fortune
Scout
Isaac Sweers (Dutch)

Other ships sunk in Colombo by 1st Carrier Airfleet attack on April 5th , 1942:
Aux. Cruiser Hector*
DD Tenedos*
 

Sources:
1. Japanese Cruisers of the Pacific War by E. LaCroix & H. Wells II.
2. Chronology of the War at Sea 1939-1945 by J. Rohwer & G. Hummelchen
3. A Battle History of the Imperial Japanese Navy (1941-1945) by P. Dull
4. Nihon Kaigun website