Unfortunately many boats have Aluminium storage water tanks - certainly many Aluminium boats also have integral tanks made of Aluminium.
In the filtration stage of the purification of water Aluminium in the form of
Sulphate: the common form being Al2(SO4)3•18H2O. Aluminium sulphate is mainly used in water treatment , being widely applied as a coagulant for clarification of water for industrial and drinking purposes. What it does is to flocculate the solid small particles into larger lumps in order that these may be trapped in the initial filter stage - prior to disinfecting and taste removal stages.
This web site provides information of the question of drinking water that has been treated with Aluminium Sulphate :-
It is almost impossible to avoid ingesting aluminium which is bioavailable all around us - aluminium is the Earth's third most common element and occurs naturally in food and water. Most of the aluminium we consume in our food and drinking water is not absorbed and goes straight through our bodies to be excreted in faeces.
By the way the jury is still out on Alzheimer and Aluminium being a contributer to the affliction.
As an aside - Yachties should keep a supply of Aluminium Sulphate powder in their medical kit - for treating stings from jellyfish venom
: add the powder to water in a ratio of about one to one in volume -- apply to the affected area -- wash off after 1/2 hour or so -- apply again if necessary. A good scientific paper on treating the stings :- http://www.mja.com.au/public/issues/184_07...ot11135_fm.html