You need to learn the Planar Summons (Water Elemental) Spell
Prerequisites
And
Or
You also need $500 of Magically/Alchemically Enhanced Insenses and Oils associated with Purity of Water.
You also should use a Magic Circle or Medicine Lodge to ensure you don't get outside influences disrupting your summoning
Lesser Water Elemental
This is a basic Water Elemental with Minimal Attributes, Total 50 points (Cost 5)
#Attributes: ST 8 [-20], DX 11 [+20], IQ 8 [-40], HT 7 [-30]
#Secondary Characteristics: SM -2, Speed 5, Move 5, Dodge 8 Base Melee Damage = Thr:1d+1, SW: 2d+1
#Advantages: Amphibious [+10], Chameleon 1 [+5], Immunity to Metabolic Hazards [+30], Injury Tolerance (Diffuse) [+100], Pressure Support 2 [+10] Slippery 4 [+8], Inate Attack (Drowning):{Melee (Close), Side-Effect (Stun), L1(1d6) [+12/Level]. Inate Attack (Water Jet): {Jet, Double Knockback} L1(1d6) [+6/Level]
#Disadvantages: Vulnerability (Fire Based Attacks; x2) [-20] Staid [-1], Fragile (Unnatural) [-50]
Faster Water Elemental
This is a more powerful, and more costly Water Elemental, Total 80 points (Cost 8)
#Attributes: ST 8 [-20], DX 12 [+40], IQ 8 [-40], HT 8 [-20]
#Secondary Characteristics: SM -2, Speed 6 [+10], Move 6, Dodge 9 Base Melee Damage = Thr:1d+1, SW: 2d+1
#Advantages: Amphibious [+10], Chameleon 1 [+5], Immunity to Metabolic Hazards [+30], Injury Tolerance (Diffuse) [+100], Pressure Support 2 [+10] Slippery 4 [+8], Inate Attack (Drowning):{Melee (Close), Side-Effect (Stun), L1(1d6) [+12/Level]. Inate Attack (Water Jet): {Jet, Double Knockback} L1(1d6) [+6/Level]
#Disadvantages: Vulnerability (Fire Based Attacks; x2) [-20] Staid [-1], Fragile (Unnatural) [-50]
The above 2 are just examples, the costs can be tweaked within reason. Attributes have no limits except for Power Levels, same with the attack and Striker ST, adding HP, Speed, Move and Enhanced Dodge are viable within limits.
The Drowning Innate Attack is resolved as a Grappling Attack followed by the Drowning, the Water Elemental projects some of its form and surrounds the Head and begins to drown the victim, Follow Suffocation Rules on Campaigns Page 436-437