Update CPT post meta with update_post_meta and cron job

Your problem is caused by your code… Let me comment it line by line, so the problem is more visible…

global $post;  // <- great, you use global $post variable

$comp_args = array( // <- you set params for your query
    'post_type'=>'boat-company',
    'posts_per_page'=>'-1',
    'post_status '=> 'publish',

);

$posts_array = get_posts( $comp_args );  // <- you run the query

if ($posts_array) {  // you check if there are any posts
    foreach( $posts_array as $post_id ) { // and if there are any posts, you loop through them and (but not in a correct way)...

        // but this (next) line doesn't do anything... The global $post variable won't be set
        // during Cron call... So this line won't work as you want it to...
        setup_postdata($post);

        // ... and that means that you can't get ID from $post, so $company_id won't contain any ID
        $company_id = $post_id->ID;

        $company_rating = 42;

        // so this line won't work as well...
        // also it would change only those meta fields that were containing "true", so...
        update_post_meta( $company_id, 'company_rating', $company_rating, true );
    }
}

wp_reset_postdata();  // you don't have to reset query in here, because you haven't changed any global query...

So here’s the cleaner way to do what you want:

function update_company_ratings_test() {    
    $comp_args = array( 
        'post_type'=>'boat-company',
        'posts_per_page'=>'-1',
        'post_status '=> 'publish',    
        'fields' => 'ids'
    );

    $posts_array = get_posts( $comp_args );

    foreach( (array)$posts_array as $company ) {    
        $company_rating = 42;
        update_post_meta( $company->ID, 'company_rating', $company_rating );
    }
}
add_action( 'update_cron_test', 'update_company_ratings_test' );

if ( !wp_next_scheduled( 'update_cron_test' ) ) {  
    wp_schedule_event( time(), 'hourly', 'update_cron_test' );  
}