Water skiing is observed to be a risk for ALS and water skiing often results in swallowing large quantities of lake water by mistake, from wipeouts or from poor ability to get up on the skis when the boat begins to accelerate... If cyanobacteria can reside in pig intestine, why not in human intestine?

The growth of cyanobacteria in Human gut is actually quite feasible, as explained in this reference: Survival, growth and toxicity of Microcystis aeruginosa PCC 7806 in experimental conditions mimicking some features of the human gastro-intestinal environment.Chem Biol Interact. 2014 May 25;215:54-61.

Relevant references:

Water skiing risk cited: Environmental and Occupational Exposures and Amyotrophic Lateral Sclerosis in New EnglandNeurodegener Dis. 2017;17(2-3):110-116.

Cyanobacteria seen in pig intestine:

Early Postnatal Diets Affect the Bioregional Small Intestine Microbiome and Ileal Metabolome in Neonatal Pigs.J Nutr. 2017 Aug;147(8):1499-1509.

Survival, growth and toxicity of Microcystis aeruginosa PCC 7806 in experimental conditions mimicking some features of the human gastro-intestinal environment.Chem Biol Interact. 2014 May 25;215:54-61.

"Our results show new features of CB: a significant resistance of M. aeruginosa in conditions far from its optimal one, that is an environment mimicking some of the important characteristics of human gastrointestinal tract, suggesting the possibility of an internal source of exposure to CTX, with implications for the risk assessment."

A cluster of amyotrophic lateral sclerosis in New Hampshire: a possible role for toxic cyanobacteria blooms.Amyotroph Lateral Scler. 2009;10 Suppl 2:101-8. Assessing Cyanobacterial Harmful Algal Blooms as Risk Factors for Amyotrophic Lateral Sclerosis.Neurotox Res. 2018 Jan;33(1):199-212.