Don't base your opinion of the crab on the Navalon at all. To say the Navalon costs 5 stars is extremely misleading and I will get to that. The crab should be compared to its nearest cousins, the dragon and the giant. I will, however, get back to the Navalon too. Pardon the long reply, but as someone with well over 100k in perfection with all tribes (including the "crab accursed" Aquarion), I think I have something to add to this.
1. Comparison to Dragon: Now I would argue that the dragon is superior due to range, speed and splash damage, but you have to consider that it has a very long incubation period which can be EXTREMELY important and that during the egg period it is defenseless, with a low defense, slow speed and NO attack. Even once fully matured it still has low health compared to other super units. Overall I prefer the dragon to every unit in the game at this point, though.
2. Comparison to Giant: The giant, while having a greater attack than the crab, is also very very slow. The crab has the potential to move nearly twice as fast as the giant in densely populated areas due to the escape skill and the crab can swim. IMO: crab is slightly better if not for the fact that you cannot turn it into a battleship. I'll call it a draw.
3. Comparison to Navalon: At it's simplest, the Navalon is not costing 5, but 15, because you can no longer harvest the whale! However, the comparison gets worse, because you are FORCED to forfeit harvesting whale even if you have absolutely no use in a navalon (small ponds that dont connect, late game with enemies nearly defeated, cities too far from shore, etc).
I would say in a typical game that I harvest 4-5 whales, and probably only 2-3 navalons would really be usable due to the reasons above. Its entirely possible that you forfeit 4 whales that all other tribes would be able to harvest and only get 2 navalons in usable locations/timing. In this situation your Navalons are costing 30 stars each (possibly more). I would say that, on average, they will have an effective cost of 20-23 stars when counting forfeited whales.
In a head to head battle, the crab beats the Navalon. The Navalon will take as much damage in attacking as the crab does in defending, but the crab wont use a turn and has more health. In the water it is true that the Navalon beats the crab overall... by far. The Navalon is faster and can take down multiple small units in a turn with persist, but it also cannot go on land, conquer cities, etc. If I know Im in a major sea battle, give me the Navalon. For versatility and ability to conquer land and cities, I'll take the crab. That makes the crab approximately equal to a unit that effectively costs around 23 stars (but ranging from 15-35 depending on how many whales you are now unable to harvest or effectively use as Navalons)
4. Even if it is true that the crab is slightly underwhelming compared to other super units, as I used to think it was as well, this comes with the trade off that the tridenton is easily the best purchasable unit. Overall the Aquarion, while not my favorite tribe, have quite a few fans and people who consider them the most powerful tribe. To make the crab more powerful would unbalance the tribe as a whole without also reducing the effectiveness of the tridenton.
If the crab is tweaked as proposed, shouldn't the Giant also be tweaked as they are nearly equal on land (Giant having +1 attack but Crab having escape) and it takes Navigation and 20 stars to make the Giant better than the crab in water. Perhaps a better idea would be to have a paid upgrade to the crab for said improvements. Say... 6 stars to give it dash and scout or 8 stars with dash and persist (which would now make it an approximate equal to the Navalon in water and the Giant on land, easily the best unit in the game (possibly Dragon aside)