Both sisal and seagrass carpets are hygroscopic. That is, they absorb moisture and give off moisture, depending on the relative humidity in their immediate environment. Although humidity is good for natural fiber carpets, saturation of the fibers with water can cause undesirable dimensional change. Therefore, steam-cleaning, wet shampooing, or any other method that involves water saturation is NOT recommended! For overall cleaning, spray-extraction method is recommended, using minimum moisture setting on the machine. Here the cleaning liquid is applied by pressure and removed by vacuuming in the same working cycle. Application should be continuous, keeping the application moving to prevent excess moisture in any spot. Note that complete moisture extraction is not possible. The moisture penetrates the fibers and the coarse structure causes the absorption of air.
No water marks are visible with this method due to the even application of moisture. If the carpet has been glued down, it should be determined that the adhesive is not water soluble before applying moisture. It should be noted that when moisture is applied to Seagrass it can revert to its natural colouring this may be whats happened in your case, the water stain may well be the natural colour.
If so and in a restoration mode, it might be possible to 'even' up the colouring by applying moisture as outlined above to blend the colour in over the entire piece of carpet, this must be qualified with the client first.

Obey the golden rule when it comes to natural floor coverings; if in doubt,
Dont Touch !