Last I checked, Water Bolt was recommended for an uptick, so I seconded its current position - A+ material on account of its usability when you're intended to get one, but not S-tier on account of how unreliable it is to acquire before then (relying purely on worldgen to give you one pre-Skeletron). I suppose removing the contention point for now would work (unless someone else contests it in the future).
Overall, I feel the reliability of acquiring a weapon is one thing that makes or breaks its standing; the best weapons are both powerful and reliable/easy to acquire, and good weapons can be tragically weighed down by their rarity. The Arkhalis in particular is a
fantastic weapon pre-hardmode, to the point that I believe it'd be a contender for the low/mid A tier if judging only its performance. Unfortunately, the downright atrocious odds of getting one even when you're hunting for it (the already sparse existence of sword pedestals, the still rare sword shrines, the 2 in 3 chance that any pedestal is fake, and
then a 9 in 10 chance that you don't get the Arkhalis from a real pedestal) leaves it teetering right on the border of B-/C+ to me. Players can, have, and will likely continue to scourge entire worlds and walk away without a single Arkhalis in their hands.
Similarly, the Slime Staff is more of a B- weapon to me. Unless you've beaten Skeletron and can set up 1sec timers to AFK your way to a Slime Staff, you're likely resorting to macros or spacebar mashing for hours on end in order to get the single weakest minion in the game (and don't forget you still need to go spelunking for a Slime Statue and most likely a bucket of lava to begin with). Again, if it were any more accessible, it'd be more of a B/B+ weapon, but as it is...
Star Cannon is hardly difficult to acquire and operate, but using it for any longer than a few seconds means spending every night scouring the surface for ammo. It's a choice weapon against the Wall of Flesh and even holds up against the Destroyer when you can line up your shots, but the tedium of collecting Fallen Stars just makes it not worth using on almost anything else. Overall, B- sounds about right.
As for Wand of Sparking... I actually can't pass much judgement on it simply because I have never, ever gotten my hands on it before finding a better magic weapon (though that might count for something in and of itself). That said, the Terraria wiki has a surprisingly good writeup on it which may be worth considering:
Mana regeneration scales based on both the player's current and maximum mana. Regenerating 20 mana if the player only has the starting cap of 20 will take significantly longer than it would to fully recover the naturally upgraded cap of 200. Use of the Wand of Sparking as a primary weapon in place of other early weapons is nearly impossible, as casting with the Wand before consuming any Mana Crystals will allow for only a few shots per minute, and killing a single zombie will nearly deplete a starting player's mana pool.