Display the link of a custom taxonomy

In your code you are just storing an “array” in the variable $themeOne. The code you should be using to make the query is:

$themeOne = new WP_Query(array(
'tax_query' => array(
    array(
        'taxonomy' => 'event-themes',
        'terms' => 'boat-theme'
    )
)
));

The result should be an array of objects. You can then access these using the following code:

foreach($themeOne as $term){
    print $term->slug;
}